Google found:
d) If you have error like
It means you used an unofficial translation of our game, which damaged your persistent files! You should find and delete all persistent files on your PC to fix it. They are located in saves folders, and paths to save folders are described above.
e) Make sure files are not corrupted. Try to download the game again, and if possible put it on another drive, even if logical.
f) Make sure your antivirus doesn't interfere with the game. Sometimes AVs settings are too strict. Try to add the game to AV exceptions.
g) Try to launch the game with admin privileges.
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From another source - this is sign of a missing file
Where have you downloaded FET from?