When you launch zwift a window opens and down the bottom of that window it should say a number, From memory I think it should be 1.0.49821. If not make sure to update the zwift app on the iMac.
What Mac OS are you using? 10.15?
If so there’s a small chance zwift doesn’t have permission to write to the documents folder and you need to manually allow it.
See the post here about how to try to do that (if you don’t have any 30 day personal bests or any custom workouts then there are less steps).