This error means a transaction is fully or partially matched in a statement in the Auto Bank Reconciliation (ABR) function. To resolve it, open the Auto Bank Reconciliation function and work through each statement to locate the transaction. Either remove or clear the match, or delete the statement if it is no longer needed. You are looking for lines in the left-hand panel with a status of 'C' (fully matched) or 'P' (partially matched).
