This issue has been resolved with Garmin Express release 6.10.0. Please download it from the link below. If issues persist, please perform a FULL uninstall of Garmin Express, reboot the Mac, and reinstall the application.
Garmin Express for Mac 6.10.0
Full Uninstall of Garmin Express
Then you'll need to contact Product Support for help for some detailed troubleshooting of your system ...