Hi there, @Baris8182
I will try my best to help you
1) Verify ETS2 assessment. (Steam - ETS 2/ATS2 - Properties - Local Files - Verify Integrity of Local Files)
2) Try to install the Launcher to C:\ProgramData. (Settings -> TruckersMP installation directory -> try to install to C:\ProgramData)
If still not working, we need to uninstall Launcher from computer.
1) Win+R -> %programdata% -> Delete TruckersMP folder.
2) Delete the folder named C:\Program Files\ - TruckersMP Launcher.
3) Delete the My Documents - ETS2MP and ATS2MP folders.
4) Win+R - regedit - expand HKEY_LOCAL_MACHINE - expand SOFTWARE - delete TruckersMP file.
After doing these, download the Launcher again https://truckersmp.com/download
i hope i solved your problem good night