Any help please?
I don't know. Maybe try putting /usr/lib64/qt6 in quotes?

Qt Code: 
  1. qmake6 -set QT_INSTALL_PREFIX "/usr/lib64/qt6"
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Edit:
I only get "/usr" but not the complete path as written above. This seems strange to me. I expected to see "/usr/bin64/qt6"
Now wait - what is it, /usr/lib64/qt6 or /usr/bin64/qt6? If there is no Qt installation at whatever you tried, then it could fail.