Thank you lamoda and Oliver Stöneberg. I didn't see the cmake output listing all the options as the configuration halted before that part. My cmake experience was not good enough to know that I should have run a basic config in order to see the other rules; the docs listed BUILD_GUI so I thought that was enough. My build was successful after adding -DCMAKE_PREFIX_PATH="/home/user/Source/Qt/6.6.2/gcc_64/lib/cmake/Qt6"
If I try to use qmake, local install: ~/Source/Qt/6.6.2/gcc_64/bin/qmake Project ERROR: Qt 6 is not supported via qmake - please use CMake instead OK, I will use cmake: mkdir build; cd build cmake -DBUILD_GUI=ON -DHAVE_RULES=ON .. ... CMake Warning at cmake/findDependencies.cmake:27 (message): Building with Qt5 is deprecated (it went EOL in May 2023) and will be removed in a future release - please use Qt6 instead (fail) I'm not experienced with cmake; - I need to tell cmake where to find my Qt6...