Are you sure you do not have a place or other โstrangeโ characters in your path to the project? If so, then Qt is unhappy.
You can also try to delete the *.pro.user file and try to compile it again. If, unfortunately, none of them work, you can try to compile your project on the command line.
cd to the root directory of your project and:
qmake -projectmingw32-make
If this still cannot compile your project, you should check your Qt installation.
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