This refers to the problems described at the following link:

https://forum.qt.io/topic/138533/qt-creator-6-7-8-qt-6-qmake-doesn-t-exist-isn-t-executable-or-has-no-output

It seems that Qt Creator 6 and up cannot run qmake in order to find the installed Qt Kit. 

1) The console error output is the following after a long wait period.

Cannot update Qt version information from /home/USER/Qt/6.3.1/gcc_64/bin/qmake: "/home/USER/Qt/6.3.1/gcc_64/bin/qmake" produced no output: .. 

2) Setting QTC_USE_QPROCESS=1 speeds up the start of Qt Creator but with the same error.

3) Qt Creator fails to load the CMake project, only showing CMakeLists.txt.

4) Preferences > Kits > Qt Versions > Auto-detected shows qmake installed at the correct path but unusable with the error "qmake does not exist or is not executable"

5) qmake does exist at the path shown and is executable

The only version of Qt Creator that still works is 5.0.3 using Qt 5.15.2 / GCC 7.3.1 20180303 / Red Hat 7.3.1-5