It could be due to a number of reasons. Begining with the missing permissions for Device Upload and/or Data Storage & Processing in GC User Privacy Setting, over dysfunctional web or network connection of your phone, missing application permissions, misconfigured phone, and many others. Check all of it, study the documents available on the Support webiste of Garmin (some are listed below), and also have a look at tips in other similar threads here on the forum.
