Geant4 should build with Qt 5.12 but…could you try updating your Qt installation. Stay with Qt5 (do not install Qt6) - the latest is Qt 5.15.2, I think (which is what I am using on my Apple Mac).
It’s also possible that the wrong Qt binaries are installed - the first lines are warning about “Required feature process environment for file …”. Checking that indicates that this may be an incompatibility between the Qt binaries and the Windows system. The path for Qt is
C:\Qt\Qt5.12.11\Qt5.12.11\winrt_x64_msvc2017, and I think this should be
C:\Qt\Qt5.12.11\Qt5.12.11\msvc2017 or more likely
If those aren’t present, you’ll need to reinstall the appropriate package from Qt for these (they are usually listed on the download page or the installer will prompt.