"QMainWindow: No such file or directory" error: Compiling using qmake only.

[robgeek]

I'm trying to compile a qt project using terminal (Linux) qmake, but I'm getting the follwoing error message:
I tried to follow this manual: https://doc.qt.io/qt-5/qmake-manual.html

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <QMainWindow>
 
QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
 
public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();
 
private:
    Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private:
    Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H

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mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
 
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}
 
MainWindow::~MainWindow()
{
    delete ui;
}

#include "mainwindow.h"
#include "ui_mainwindow.h"

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

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

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main.cpp

#include "mainwindow.h"
 
#include <QApplication>
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

#include "mainwindow.h"

#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

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main.pro

HEADERS += mainwindow.h
SOURCES += mainwindow.cpp
SOURCES += main.cpp

HEADERS += mainwindow.h
SOURCES += mainwindow.cpp
SOURCES += main.cpp

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[rob@archrob qteste]$ qmake main.pro
[rob@archrob qteste]$ make
g++ -c -pipe -O2 -march=x86-64 -mtune=generic -O2 -pipe -fno-plt -Wall -Wextra -D_REENTRANT -fPIC -DQT_NO_DEBUG -DQT_GUI_LIB -DQT_CORE_LIB -I. -isystem /usr/include/qt -isystem /usr/include/qt/QtGui -isystem /usr/include/qt/QtCore -I. -I/usr/lib/qt/mkspecs/linux-g++ -o mainwindow.o mainwindow.cpp
In file included from mainwindow.cpp:1:
mainwindow.h:4:10: fatal error: QMainWindow: No such file or directory
4 | #include <QMainWindow>
| ^~~~~~~~~~~~~
compilation terminated.
make: *** [Makefile:663: mainwindow.o] Error 1