This error usually appears when Anchor tries to sync a transaction (such as an invoice) that is dated within a period already locked in your Xero account.
To resolve this:
In Xero, go to Accounting > Advanced
Click Financial Settings
Under Lock Dates, remove or adjust the date
Click Save
Return to Anchor and retry the sync
Once the sync is successful, you can re-enable the lock date in Xero if needed.