I am sorry for the troubles. Please see the information in the the articles below.
What should I do if I receive a server error while using Garmin Express?
https://support.garmin.com/support/searchSupport/case.faces?caseId={dd8e8b20-ef1e-11e2-7dd9-000000000000}
What can I try when my ANT Fitness device will not sync with Garmin Express?
https://support.garmin.com/support/searchSupport/case.faces?caseId={600f3dd0-3e6a-11dd-d7af-000000000000}
If you continue to have troubles I suggest you contact Garmin Product Support by phone.