i had the same issue....after copy "City Navigator Europe NT 2020.10.gmap" folder to hidden folder "C:/ProgramData/Garmin/Maps/" you need select on folder "City navigator Europe NT 20xx.xx.gmap" properties (in General tab) and deselect "Read-only" then hit "Apply"
after that you can run "UnlockMapSourcedir.exe" again.
goodluck.
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