I'm using qtgreato,r with the compilers noted in my last post, to build the project from it's .pro file.
Your last post says you are using CLang 11 for C++ and your choice of Qt qmake says you are using GCC.
So, when you build your project, exactly how is the compiler invoked?
Still unanswered but the next step will make this obvious.
If there is another way to build this project without using qtcreator?
Sure.mkdir /tmp/build_dir cd /tmp/build_dir /home/XXX/qt/6.6.1/gcc_64/bin/qmake /path/to/source/project.pro make
(Exactly the same sort of steps that Qt Creator takes on your behalf).