Thanks for your question.
To get the exact and meaningful error or to know root cause of an issue, you can run
Azure SQL Data Sync Health Checker report
This PowerShell script will check if all the metadata of a hub and member is in place and also validate the scopes against the information we have on the sync metadata database (among other validations). This script will not make any changes, will just validate data sync and user objects. It will also gather some useful information for faster troubleshooting.