@MountBiker
Your GPSMAP 64s runs firmware which contains MSV protection (MSV is Map Signature Verification). Patching for JNX only allows the use of Birdseye data without a current subscription. If you are trying to use an unlocked/unofficial map *.img file and see a message on startup saying "Can't Authenticate Maps ..." then the firmware needs additional patching. You won't get help with that here because of the already stated forum position regarding unofficial maps and manipulation of firmware to allow use of illegal mapping. Search the wider internet for "Universal Firmware Patcher for Protected Garmin Devices" and look for the latest version possible, that's V2.20 afaik. If it doesn't work for the V6.10 firmware you may need to additionally downgrade to an earlier patched fw, maybe to V5.60.

Of course the proper thing to do is to buy genuine mapping data and a BirdsEye subscription from Garmin for your device. We don't encourage piracy here at all.