Can't create a new Qt project (qmake not working)

[Gabriele]

Hi, i have a big problem when i create or open a project in Qt Creator. I installed several times qt but nothing seems changing. Right now i'm working with Qt 5.15.2 and basically when i create a new project, in the project window appears only the .pro file, but the source files are not included. When i run the qmake or try to build, several errors appear:

C:/Qt/5.15.2/mingw81_64/mkspecs/features/toolchain.prf:76: Variable QMAKE_CXX.COMPILER_MACROS is not defined.
Project ERROR: failed to parse default search paths from compiler output
10:31:20: The process "C:\Qt\5.15.2\mingw81_64\bin\qmake.exe" exited with code 3.
Error while building/deploying project TEST (kit: Desktop Qt 5.15.2 MinGW 64-bit)
When executing step "qmake"

In manage kits, Kits, Qt Versions and Compilers aren't giving any problems. Can someone help me?
Thanks