Why the Sync Error occurs
This error occurs when the transaction cannot sync to Xero because the date of the transaction is within a locked period.
How to Fix the Sync Error
For a locked period, from the Sync Status page select I've updated this Bill in Xero; remove this error and sync future updates. then make any necessary changes in Xero. This is the recommended option.
Open the locked period and sync again to allow the transaction to sync over, however any other changes made to that period may also sync.