Regards, Vlado

import gui
import logwindow
import sys
import os

from PyQt5 import QtCore, QtGui, QtWidgets
from PyQt5.QtWidgets import QMainWindow, QApplication
from PyQt5.QtCore import QTimer
import time

#------------------------------------------------------------------------------
def userName():
    un = login.lineEdit_user_name.text()
    print(un)
    return un

def passWord():
    pw = login.lineEdit_password.text()
    print(pw)
    return pw

def log_IN():
    userName()
    passWord()

def show_login_window():  
    log_IN()
    print("-------")
#-----------------------------------------------------------------------------
def push_buttons():
    master.pushButton_call_window.clicked.connect(master.open_Login_Window)
    login.pushButton_Login.clicked.connect(show_login_window)
#---------------------------------------------------------------------------------
def triger():
    push_buttons()  


if __name__ == "__main__":
    app = QtWidgets.QApplication(sys.argv)
    gui_master = gui.QtWidgets.QWidget()
    master = gui.Ui_Form()
    master.setupUi(gui_master)

    gui_login = logwindow.QtWidgets.QDialog()
    login = logwindow.Ui_Dialog()
    login.setupUi(gui_login)
    
    timer = QTimer()
    timer.timeout.connect(triger)  
    timer.start(1000)

    gui_master.show()
    gui_login.show()
    sys.exit(app.exec_())

gui.py

from PyQt5 import QtCore, QtGui, QtWidgets

from logwindow import Ui_Dialog

class Ui_Form(object):
    def open_Login_Window(self):    
        self.window = QtWidgets.QDialog()
        self.ui = Ui_Dialog()
        self.ui.setupUi(self.window)        
        self.window.show()
        
    def setupUi(self, Form):
        Form.setObjectName("Form")
        Form.resize(400, 300)
        self.pushButton_call_window = QtWidgets.QPushButton(Form)
        self.pushButton_call_window.setGeometry(QtCore.QRect(130, 130, 141, 30))
        self.pushButton_call_window.setObjectName("pushButton_call_window")

        self.retranslateUi(Form)
        QtCore.QMetaObject.connectSlotsByName(Form)

    def retranslateUi(self, Form):
        _translate = QtCore.QCoreApplication.translate
        Form.setWindowTitle(_translate("Form", "Form"))
        self.pushButton_call_window.setText(_translate("Form", "Call Login Window"))


if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    Form = QtWidgets.QWidget()
    ui = Ui_Form()
    ui.setupUi(Form)
    Form.show()
    sys.exit(app.exec_())

logwindow.py

# -*- coding: utf-8 -*-

# Form implementation generated from reading ui file 'logwindow.ui'
#
# Created by: PyQt5 UI code generator 5.15.2
#
# WARNING: Any manual changes made to this file will be lost when pyuic5 is
# run again.  Do not edit this file unless you know what you are doing.


from PyQt5 import QtCore, QtGui, QtWidgets


class Ui_Dialog(object):
    def setupUi(self, Dialog):
        Dialog.setObjectName("Dialog")
        Dialog.resize(400, 300)
        self.pushButton_Login = QtWidgets.QPushButton(Dialog)
        self.pushButton_Login.setGeometry(QtCore.QRect(140, 200, 91, 30))
        self.pushButton_Login.setObjectName("pushButton_Login")
        self.lineEdit_user_name = QtWidgets.QLineEdit(Dialog)
        self.lineEdit_user_name.setGeometry(QtCore.QRect(130, 70, 113, 30))
        self.lineEdit_user_name.setObjectName("lineEdit_user_name")
        self.lineEdit_user_name.setText("user name")
        self.lineEdit_password = QtWidgets.QLineEdit(Dialog)
        self.lineEdit_password.setGeometry(QtCore.QRect(130, 120, 113, 30))
        self.lineEdit_password.setObjectName("lineEdit_password")
        self.lineEdit_password.setText("password")

        self.retranslateUi(Dialog)
        QtCore.QMetaObject.connectSlotsByName(Dialog)

    def retranslateUi(self, Dialog):
        _translate = QtCore.QCoreApplication.translate
        Dialog.setWindowTitle(_translate("Dialog", "Dialog"))
        self.pushButton_Login.setText(_translate("Dialog", "Login"))


if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    Dialog = QtWidgets.QDialog()
    ui = Ui_Dialog()
    ui.setupUi(Dialog)
    Dialog.show()
    sys.exit(app.exec_())

def open_Login_Window(self):    
        self.window = QtWidgets.QDialog()
        self.ui = Ui_Dialog()
        self.ui.setupUi(self.window)        
        self.window.show()  
        ...
        name = self.ui.user_Name()
        password = self.ui.pass_Word()

You also should use exec() instead of show() I think.

main.py (gui_91.py is imported here)

import RPi.GPIO as GPIO
from subprocess import call
import gui_91
import sys
import os
from datetime import datetime
from slaves import *
from Buffer_script_02 import *
from graph_08c import *  
from kalkulacije import *
from xls_store_8 import*

from PyQt5 import QtCore, QtGui, QtWidgets
from PyQt5.QtWidgets import QMainWindow, QApplication
from PyQt5.QtCore import QTimer
import time

...

gui_91.py (login_window.py is imported here)

from PyQt5 import QtCore, QtGui, QtWidgets

from login_window import Ui_Dialog

try:
    _fromUtf8 = QtCore.QString.fromUtf8
except AttributeError:
    def _fromUtf8(s):
        return s

try:
    _encoding = QtGui.QApplication.UnicodeUTF8
    def _translate(context, text, disambig):
        return QtGui.QApplication.translate(context, text, disambig, _encoding)
except AttributeError:
    def _translate(context, text, disambig):
        return QtGui.QApplication.translate(context, text, disambig)

class Ui_SCADA(object):
    
    #-----------------------------------------------------------------------------------------------
    def open_Login_Window(self):    
        self.window = QtWidgets.QDialog()
        self.ui = Ui_Dialog()
        self.ui.setupUi(self.window)        
        self.window.show()  
    #-----------------------------------------------------------------------------------------------
        
    def setupUi(self, SCADA):
        SCADA.setObjectName("SCADA")
        SCADA.resize(888, 665)

...

login_window.py (I want to use methods "user_Name" and "pass_Word" in main.py or in gui_91.py)

# -*- coding: utf-8 -*-

# Form implementation generated from reading ui file 'login.ui'
#
# Created by: PyQt5 UI code generator 5.15.2
#
# WARNING: Any manual changes made to this file will be lost when pyuic5 is
# run again.  Do not edit this file unless you know what you are doing.


from PyQt5 import QtCore, QtGui, QtWidgets

try:
    _fromUtf8 = QtCore.QString.fromUtf8
except AttributeError:
    def _fromUtf8(s):
        return s

try:
    _encoding = QtGui.QApplication.UnicodeUTF8
    def _translate(context, text, disambig):
        return QtGui.QApplication.translate(context, text, disambig, _encoding)
except AttributeError:
    def _translate(context, text, disambig):
        return QtGui.QApplication.translate(context, text, disambig)

class Ui_Dialog(object):
    def user_Name(self):
        un = self.lineEdit_user_name.text()
        print(un)
        return un

    def pass_Word(self):
        pw = self.lineEdit_password.text()
        print(pw)
        return pw
    
    def log_IN(self):
        print("Log In")
        self.user_Name()
        self.pass_Word()
        
    def setupUi(self, Dialog):
        Dialog.setObjectName("Dialog")
        Dialog.resize(400, 300)
        self.bgwidget = QtWidgets.QWidget(Dialog)
        self.bgwidget.setGeometry(QtCore.QRect(-1, -1, 401, 301))
        self.bgwidget.setStyleSheet("QWidget#bgwidget{\n"
"background-color: qlineargradient(spread:pad, x1:0, y1:0, x2:1, y2:1, stop:0 rgba(0, 0, 0, 255), stop:1 rgba(255, 255, 255, 255));}")
        self.bgwidget.setObjectName("bgwidget")
        self.label = QtWidgets.QLabel(self.bgwidget)
        self.label.setGeometry(QtCore.QRect(160, 30, 91, 31))
        font = QtGui.QFont()
        font.setPointSize(20)
        self.label.setFont(font)
        self.label.setStyleSheet("color: rgb(255, 255, 127);")
        self.label.setObjectName("label")
        #self.pushButton_Log_in = QtWidgets.QPushButton(self.bgwidget)
        self.pushButton_Log_in = QtWidgets.QPushButton(self.bgwidget, clicked = lambda: log_IN())
        self.pushButton_Log_in.setGeometry(QtCore.QRect(140, 230, 121, 30))
        self.pushButton_Log_in.setStyleSheet("color: rgb(255, 255, 127);\n"
"background-color: rgb(99, 99, 99);")
        self.pushButton_Log_in.setObjectName("pushButton_Log_in")
        self.lineEdit_user_name = QtWidgets.QLineEdit(self.bgwidget)
        self.lineEdit_user_name.setGeometry(QtCore.QRect(140, 110, 121, 30))
        self.lineEdit_user_name.setObjectName("lineEdit_user_name")
        self.lineEdit_password = QtWidgets.QLineEdit(self.bgwidget)
        self.lineEdit_password.setGeometry(QtCore.QRect(140, 170, 121, 30))
        self.lineEdit_password.setObjectName("lineEdit_password")
        self.label_2 = QtWidgets.QLabel(self.bgwidget)
        self.label_2.setGeometry(QtCore.QRect(60, 120, 68, 22))
        self.label_2.setStyleSheet("color: rgb(255, 255, 127);")
        self.label_2.setObjectName("label_2")
        self.label_3 = QtWidgets.QLabel(self.bgwidget)
        self.label_3.setGeometry(QtCore.QRect(60, 180, 68, 22))
        self.label_3.setStyleSheet("color: rgb(255, 255, 127);")
        self.label_3.setObjectName("label_3")

        self.retranslateUi(Dialog)
        QtCore.QMetaObject.connectSlotsByName(Dialog)

    def retranslateUi(self, Dialog):
        _translate = QtCore.QCoreApplication.translate
        Dialog.setWindowTitle(_translate("Dialog", "Dialog"))
        self.label.setText(_translate("Dialog", "Prijava"))
        self.pushButton_Log_in.setText(_translate("Dialog", "Prijava"))
        self.label_2.setText(_translate("Dialog", "Up. ime"))
        self.label_3.setText(_translate("Dialog", "Geslo"))


if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    Dialog = QtWidgets.QDialog()
    ui = Ui_Dialog()
    ui.setupUi(Dialog)
    Dialog.show()
    sys.exit(app.exec_())

Please use proper casing for your method, variable and class names. Although it doesn't matter for your machine, the humans reading your code will have a very hard time.

Follow the PEP8 document

• Variable names in lower case
• function name in snake_case, no uppercase mixed in there

Qt uses camelCase for its functions because they come from the C++ style guide lines they are using although you can change that with the latest version of PySide6.

Beside that, since as you said you are a beginner, you should take the time to read the documentation and tutorials provided by Qt's documentation. You'll learn quite a lot there to get the basics going properly.

but how to pass variable un into main?

Please explain better.
What variable and what is "main"?

# -*- coding: utf-8 -*-

# Form implementation generated from reading ui file 'login.ui'
#
# Created by: PyQt5 UI code generator 5.15.2
#
# WARNING: Any manual changes made to this file will be lost when pyuic5 is
# run again.  Do not edit this file unless you know what you are doing.


from PyQt5 import QtCore, QtGui, QtWidgets

try:
    _fromUtf8 = QtCore.QString.fromUtf8
except AttributeError:
    def _fromUtf8(s):
        return s

try:
    _encoding = QtGui.QApplication.UnicodeUTF8
    def _translate(context, text, disambig):
        return QtGui.QApplication.translate(context, text, disambig, _encoding)
except AttributeError:
    def _translate(context, text, disambig):
        return QtGui.QApplication.translate(context, text, disambig)

class Ui_Dialog(object):
    def user_Name(self):
        un = self.lineEdit_user_name.text()
        print(un)
        return un

    def pass_Word(self):
        pw = self.lineEdit_password.text()
        print(pw)
        return pw
    
    def prijava(self):
        print("Prijava")
        self.user_Name()
        self.pass_Word()
        
    def setupUi(self, Dialog):
        Dialog.setObjectName("Dialog")
        Dialog.resize(400, 300)
        self.bgwidget = QtWidgets.QWidget(Dialog)
        self.bgwidget.setGeometry(QtCore.QRect(-1, -1, 401, 301))
        self.bgwidget.setStyleSheet("QWidget#bgwidget{\n"
"background-color: qlineargradient(spread:pad, x1:0, y1:0, x2:1, y2:1, stop:0 rgba(0, 0, 0, 255), stop:1 rgba(255, 255, 255, 255));}")
        self.bgwidget.setObjectName("bgwidget")
        self.label = QtWidgets.QLabel(self.bgwidget)
        self.label.setGeometry(QtCore.QRect(160, 30, 91, 31))
        font = QtGui.QFont()
        font.setPointSize(20)
        self.label.setFont(font)
        self.label.setStyleSheet("color: rgb(255, 255, 127);")
        self.label.setObjectName("label")
        #self.pushButton_Log_in = QtWidgets.QPushButton(self.bgwidget)
        self.pushButton_Log_in = QtWidgets.QPushButton(self.bgwidget, clicked = lambda: self.prijava())
        self.pushButton_Log_in.setGeometry(QtCore.QRect(140, 230, 121, 30))
        self.pushButton_Log_in.setStyleSheet("color: rgb(255, 255, 127);\n"
"background-color: rgb(99, 99, 99);")
        self.pushButton_Log_in.setObjectName("pushButton_Log_in")
        self.lineEdit_user_name = QtWidgets.QLineEdit(self.bgwidget)
        self.lineEdit_user_name.setGeometry(QtCore.QRect(140, 110, 121, 30))
        self.lineEdit_user_name.setObjectName("lineEdit_user_name")
        self.lineEdit_password = QtWidgets.QLineEdit(self.bgwidget)
        #self.lineEdit_password = QtWidgets.setEchoMode.QLineEdit(self.bgwidget)
        self.lineEdit_password.setGeometry(QtCore.QRect(140, 170, 121, 30))
        self.lineEdit_password.setObjectName("lineEdit_password")
        self.label_2 = QtWidgets.QLabel(self.bgwidget)
        self.label_2.setGeometry(QtCore.QRect(60, 120, 68, 22))
        self.label_2.setStyleSheet("color: rgb(255, 255, 127);")
        self.label_2.setObjectName("label_2")
        self.label_3 = QtWidgets.QLabel(self.bgwidget)
        self.label_3.setGeometry(QtCore.QRect(60, 180, 68, 22))
        self.label_3.setStyleSheet("color: rgb(255, 255, 127);")
        self.label_3.setObjectName("label_3")

        self.retranslateUi(Dialog)
        QtCore.QMetaObject.connectSlotsByName(Dialog)

    def retranslateUi(self, Dialog):
        _translate = QtCore.QCoreApplication.translate
        Dialog.setWindowTitle(_translate("Dialog", "Dialog"))
        self.label.setText(_translate("Dialog", "Prijava"))
        self.pushButton_Log_in.setText(_translate("Dialog", "Prijava"))
        self.label_2.setText(_translate("Dialog", "Up. ime"))
        self.label_3.setText(_translate("Dialog", "Geslo"))


if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    Dialog = QtWidgets.QDialog()
    ui = Ui_Dialog()
    ui.setupUi(Dialog)
    Dialog.show()
    sys.exit(app.exec_())

class Ui_Dialog(object):
...
    def userName(self):
        return self.lineEdit_user_name.text()
# Same for all other text edits you need

class Ui_SCADA(object):
    def open_Login_Window(self):    
        self.window = QtWidgets.QDialog()
        self.ui = Ui_Dialog()
        self.ui.setupUi(self.window)        
        self.window.show()
        
    def setupUi(self, SCADA):
        SCADA.setObjectName("SCADA")
        SCADA.resize(888, 665)

Regards

The correct way to do this is to give your dialogs proper accessor method that will return the content of the line edits you have in there.

The widgets in your dialog are an implementation detail for the main window and thus your dialog should have a simple api that returns the data.