So I use Garmin Express on my mac.
I have uninstalled and reinstalled.
I have other Garmin devices that I have removed and re-added and it worked perfectly, by ant+ or by cable.
Garmin knows its a bug. This has happened twice to me as I accidentally bought the Garmin Edge Explore 820 first, then had to register the Edge 820 afterwards. The first time the Garmin rep on the phone reset my connect password and dealt with it herself. The second time here is what I was instructed to do (it worked).
1. Log in to Garmin connect on the web browser 2. Click the 3 line tab on the top left 3. Choose "Devices" 4. Choose the device you are having issues with 5. Click Save at the very bottom 6. Plug device into the computer and attempt Garmin Express again
Hopefully this works for you, saving you time on hold with Garmin