Try restarting your Mac. if that doesn't work download and reinstall. If that doesn't work, completely uninstall GE, then reinstall. If that doesn't work...
I have it running on a MacBook Pro (late 2013) and an iMac (late 2016)
Same major issue here! First I was in an update loop; installed update but same update message keeps appearing. I deleted GE, reinstalled : not working.
Garmin Express won't start. I deleted GE with AppCleaner, reinstalled everything : not working.
Rebooted my iMac, deleted everything, reinstalled : not working!