You can always check with Javawa Device Manager if there is something wrong.

Other possible issue is a not compatible format of the micro SD card (must be <64Gb and formatted as Fat32). The Mac’s internal programm for formatting the Sd cards is also not compatible with Garmin devices.

You can also try to copy an already prepared img map directly to the device’s garmin folder with a file manager without the need of Basecamp.

It is also recommended not to use the Basecamp version from the Mac App store but download it directly from the Garmin site:
https://www8.garmin.com/support/download_details.jsp?id=4449