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Check as to whether the maps are compatible with your device model # (whatever the maps are, whatever their source, and whatever your Garmin device's exact model # *****)
Then prepare the SD card for use. Put the maps on it, then log in to your computer's myGarmin account, click the MyMaps tab, click on the Add To Device button next to the new maps (if they are listed), and then (with some authorizations etc.) the maps will be placed on the SD card and ready for use.
for more details. If you think you have followed all those steps, then
A. did you purchase the maps through myGarmin ?
B. did you carefully determine whether the maps were compatible with your Garmin model # ?