Hi and welcome to the forum,

it's always better to post code as text using the code format button ( "</>").
Anyway, it's seems that you are not that familiar with C++ and or OOP programming languages.

There's a lot stuff going on in you code which is wrong.

• The call to Ui->cmbPorts.... in your foreach loop does not work like this, because your UI's namespace is named Ui, but not the pointer/instance to your actual compiled Ui class...

• And even then... you should only access the UI from the same class directly.
  You are in serial.cpp, while the ui, even if spelled correctly, is a private member of MainWindow. It cannot (and should not) be accessed from serial.

• Your openPort() method in serial.cpp is not even a member function of the serial class... it's just a globally declared function floating around in your serial.cpp file... which is another issue here...

• therefore the call serial1.openPort() doesn't work... because there is no serial::openPort(), as I explained above

• also your serial class is missing a constructor... we can't see your serial header, but since you are showing the code file, which contains no class/member functions at all, I doubt it's there.
  The error in your 3rd picture even says "undefined reference to serial::serial"

Might sound harsh, but I would recommend to learn the C++ basics (Objects, Pointers, Classes, Members, Inheritance and other OOP principles) and then come back before you dive further into Qt now, struggling with small things which are obligatory to work with Qt (and not get frustrated)

So to answer your question "Am I calling this method wrong?", yes and a couple more mistakes

@Comedian are you more familiar with Python?!
Have a look at the Python wrapper for Qt called "PySide"

Here are some helpful links:

• https://www.qt.io/qt-for-python
• https://doc.qt.io/qtforpython-6/
• https://wiki.qt.io/Qt_for_Python