Books are closed in Xero - Transaction date is after period lock date. You can open your books (change period lock date be before transaction date) or change the transaction date (to be after period lock date) to sync this change.
This error occurs when the transaction trying to sync is dated for a date that is before the locked period.
For a locked period, you should select "I've updated this Bill in Xero; remove this error and sync future updates." from the Sync Status page, then make any necessary changes in Xero.
Keep in mind you may open your period and sync again to allow the transaction to sync over, however any other changes made to that period may also sync. It is recommended to select for the error to clear and manually update.