I am sorry that you are having trouble with Express. We are seeing this issue when trying to add devices using computers that are set to certain region formats. We are working to correct this issue but in the meantime, you may be able to add your device to Express by following these instructions:
Click the Apple icon
Click System Preferences
Click Language & Region
Change the Region to United States under Americas (Other regions will work as well but I know this one will work)
I am sorry that you are having trouble with Garmin Express. This issue has already been resolved with a server side change. Please make sure that you have a direct connection to the internet and that the date, time and time zone are correct on the computer. If you are still experiencing this issue, please contact Product Support by phone.