I just added this "save_btn_clicked" from the Qt Designer using: right-click -> Go To Slot -> Clicked()

It can't be...The "save_btn_clicked" function is not even a slot!
That's why it doesn't work...
If you create it using designer, it should be added as a slot like this:

private slots:
    void on_save_btn_clicked();

Unless you've deleted that "private slots:" line.

Unless you've deleted that "private slots:" line.

Perfect @Bonnie ....I've deleted this line...
Thank you!

I just added this "save_btn_clicked" from the Qt Designer using: right-click -> Go To Slot -> Clicked()

It can't be...The "save_btn_clicked" function is not even a slot!
That's why it doesn't work...
If you create it using designer, it should be added as a slot like this:

private slots:
    void on_save_btn_clicked();

Unless you've deleted that "private slots:" line.

#ifndef DIALOG_H
#define DIALOG_H

#include <QDialog>

namespace Ui {
class Dialog;
}

class Dialog : public QDialog
{
    Q_OBJECT

public:
    explicit Dialog(QWidget *parent = nullptr);
    ~Dialog();

    void on_save_btn_clicked();

private:
    Ui::Dialog *ui;
};

#endif // DIALOG_H

#include "dialog.h"
#include "ui_dialog.h"

#include <QDebug>

Dialog::Dialog(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Dialog)
{
    ui->setupUi(this);
}

Dialog::~Dialog()
{
    delete ui;
}

void Dialog::on_save_btn_clicked()
{
    qDebug() << "SAVE";
}

I just added this "save_btn_clicked" from the Qt Designer using: right-click -> Go To Slot -> Clicked()

Without the code form Dialog, it's hard to guess...