The Common Ground sync is done via Salesforce API calls. This error indicates that you have reached your Salesforce limit on the number of API calls that can be done on any given day. This should resolve itself tomorrow.

If you need the data to sync today, you can call Salesforce and request a one time exception at no cost. If you have this problem regularly, you can contact Salesforce to upgrade how many API calls you can process in a day at a cost.