So I believe that the Qrubberband code posted by @SGaist needs PySide2 in order to function properly? Right?

Wrong, you did not copy my code properly and missing a self. in yours.

UPDATE: I changed everything to PyQt5 and I deleted PySide2.... and the previously mentioned error appeared:

Traceback (most recent call last):
  File "code.4.py", line 195, in mouseMoveEvent
    rubberBand.setGeometry(QRect(origin, event.pos()).normalized())
NameError: name 'rubberBand' is not defined
Aborted (core dumped)

So I believe that the Qrubberband code posted by @SGaist needs PySide2 in order to function properly? Right?

self.tb

What is tb?
It would also be helpful if you would post the relevant code.

I have this error:

File "code.py", line 231, in __init__
  empty.setSizePolicy(QSizePolicy.Expanding, QSizePolicy.Expanding)
TypeError: 'PySide2.QtWidgets.QWidget.setSizePolicy' called with wrong argument types:
PySide2.QtWidgets.QWidget.setSizePolicy(Policy, Policy)
Supported signatures:
PySide2.QtWidgets.QWidget.setSizePolicy(PySide2.QtWidgets.QSizePolicy)
PySide2.QtWidgets.QWidget.setSizePolicy(PySide2.QtWidgets.QSizePolicy.Policy, PySide2.QtWidgets.QSizePolicy.Policy)

So, I changed this line:

empty.setSizePolicy(QSizePolicy.Expanding, QSizePolicy.Expanding)

to this:

empty.setSizePolicy(PySide2.QtWidgets.QSizePolicy.Policy.Expanding, PySide2.QtWidgets.QSizePolicy.Policy.Expanding)

And then I get this error:

File "code.py", line 234, in __init__
  self.tb.addWidget(empty)
TypeError: addWidget(self, QWidget): argument 1 has unexpected type 'PySide2.QtWidgets.QWidget'

what I am supposed to do? If I fix the one, the other becomes bug and vice-versa.... I mean what should change here?

export QT_DEBUG_PLUGINS=1
./YOUR_APP

And post the output here.

QObject::moveToThread: Current thread (0x2b4f9a0) is not the object's thread (0x37b2280).
Cannot move to target thread (0x2b4f9a0)

qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "/home/john/Desktop/venv/lib/python3.8/site-packages/cv2/qt/plugins" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: xcb, eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, wayland-egl, wayland, wayland-xcomposite-egl, wayland-xcomposite-glx, webgl.

Aborted (core dumped)

Then I followed this solution: https://forum.qt.io/topic/125009/problem-with-qt-qpa-plugin-running-on-a-docker-in-kubuntu/18 and basically I installed these libraries (all with sudo):

pip3 install opencv-python-headless
apt-get update ##[edited]
apt-get install ffmpeg libsm6 libxext6  -y
pip3 install matplotlib
apt-get install python3-pyqt5 -y
pip3 install pyqt5
pip3 install wheel
pip3 install pandas
pip3 install openpyxl
pip3 install geopandas
apt update -y
apt install libpq-dev -y
apt install gdal-bin -y
apt install libgdal-dev -y
pip3 install numba
pip3 install GDAL
apt-get install qt5-default -y
pip3 install numpy
ldd /usr/lib/x86_64-linux-gnu/qt5/plugins/platforms/libqxcb.so
apt-get update -y
apt-get install -y '^libxcb.*-dev' libx11-xcb-dev libglu1-mesa-dev libxrender-dev libxi-dev libxkbcommon-dev libxkbcommon-x11-dev 
apt-get install -y libxcb-util-dev
QT_DEBUG_PLUGINS=1

and I now I am getting this error:

qt.qpa.xcb: could not connect to display :1.0
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, wayland-egl, wayland, wayland-xcomposite-egl, wayland-xcomposite-glx, webgl, xcb.

Aborted (core dumped)

Any idea???

required by /home/john/.local/lib/python3.8/site-packages/PySide2/QtCore.abi3.so)

This suggests that it's not the case.

I tried this:

sudo apt-get install python3-pyqt5.qtwebengine

and this:

sudo apt-get install python3-pyqt5

I combined your above code with my code and tried to run it, but I get this error:

File "code.py", line 30, in <module>
  from PySide2.QtCore import QRect
ImportError: /lib/x86_64-linux-gnu/libQt5Core.so.5: version `Qt_5.15' not found (required by /home/john/.local/lib/python3.8/site-packages/PySide2/QtCore.abi3.so)

After google search I found and followed the suggestions from this tutorial: https://stackoverflow.com/questions/63903441/python3-importerror-lib-x86-64-linux-gnu-libqt5core-so-5-version-qt-5-15-n which does not help because it is related to PyQt5 and not PySide2.

I have to note at this point that in my code I use both PyQt5 and PySide2 packages. Could the above error be a result of conflict between the packages?

I also study this: https://forum.qt.io/topic/89263/qt5-pyqt5-and-opencv-problem-with-import

@SGaist: Thanks, I am looking at it...

Here's an example:

import sys

from PySide2.QtCore import QRect
from PySide2.QtCore import QSize
from PySide2.QtWidgets import QWidget
from PySide2.QtWidgets import QRubberBand
from PySide2.QtWidgets import QApplication

class Widget(QWidget):
    def __init__(self, parent=None):
        super().__init__(parent)

        self.rubberBand = None
        self.origin = None

    def mousePressEvent(self, event):
        self.origin = event.pos()
        if not self.rubberBand:
            self.rubberBand = QRubberBand(QRubberBand.Rectangle, self)
        self.rubberBand.setGeometry(QRect(self.origin, QSize()))
        self.rubberBand.show()

    def mouseMoveEvent(self, event):
        self.rubberBand.setGeometry(QRect(self.origin, event.pos()).normalized())

    def mouseReleaseEvent(self, event):
        self.rubberBand.hide()
        # determine selection, for example using QRect.intersects()
        # and QRect.contains().

if __name__ == "__main__":
    app = QApplication(sys.argv)
    widget = Widget()
    widget.show()
    sys.exit(app.exec_())

On a side note, it looks like you are a beginner with Python, I would recommend getting the basics first before going further.

import rubberband

but again does not work! What should I do?