Another common but infuriating Mac bug!
When using Dark Mode on Mac OSX the list of maps to install via MapManager appears to be empty (in fact it is showing white text on a white background), as shown below:
The map is listed there, it just can’t be seen. Either just click Install and the map will install fine, or turn off Dark Mode and try again, when all will be normal (black text on a white background). Don’t you just love Macs?