According to the official website of the program, it isn't advised to use the Garmin BaseCamp application from the Apple Store because it has its functions limited. To download the official version of the Garmin BaseCamp, visit its official website.
To fix your problem, make sure that your maps file has an extension that can be recognized by the Garmin software. Also, make sure that these aren't corrupted. It's advised to use only official maps that you can download from the official website.