Sounds like you are looking at City_Navigator_Europe_NTU_2023_20.gmap the folder on your PC (not the current version) used by MapSource or BaseCamp programs to create custom maps sets.
Garmin Express should have installed the gmap folder and all it's contents to C:\ProgramData\Garmin\Maps.
You will need to install 1 of the programs.
Even if you create a custom map set it will not work on your GPSMAP 64s as the gmap folder is only unlocked to the device that GE downloaded it for.
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