I installed Qt Creator from XUbuntu 20.04 repository using these commands:
sudo apt install qt5-default qtcreator qtbase5-examples qtdeclarative5-examples
And I got Qt Creator where all Qt classes are highlighted as errors with message "Use of undeclared identifier". But projects are compiled correctly. АUtocompletion of Qt Classes is also not working.
I don't want to install version from Qt site, because repo versions run faster.
Screenshot:
