It means that you probably compile using the qmake on your system instead of your qmake from the install you did and this page is very old.
The only installer is available here: http://mirrors.ukfast.co.uk/sites/qt.io/archive/online_installers/3.2/qt-unified-linux-x64-3.2.2-online.run
Install Qt5.12 or Qt5.13, please select only x64 (do no install for android and you can also remove some big parts), then make sure you can the qmake from the installed version and not the one from your system.
If I understand it correctly, I should remove the version I compiled previously, first of all, and then execute the installer. I'll check that, thanks.