on the left pane you can see that only header files are visible in the project. No other files like .cpp or .ui
are visible in qt creator (OS Linux Mint 20.1).
I don’t use Qt Creator, but it seems like there are some problems when it ran Cmake. In your “General Messages” pane in the screenshot, it doesn’t seem like it found AvogadroLibs … and “Configuring incomplete, errors occurred”
It would also help to know how you grabbed the code - I’m assuming that git downloaded all the source into avogadrolibs and subdirectories, etc.
My general sense is that Qt Creator has attempted to run cmake for the project and something failed - so it doesn’t see source targets, etc.