This error occurs when the reconciliation flag in the database shows as reconciled, but the statement is not yet reconciled in the application.
Check whether the transaction is showing as reconciled in the Enquiries screen. It may have been manually reconciled after the statement was created. If it is showing as reconciled, unreconcile it within Enquiries and try the cash book reconciliation again.
If this does not resolve the issue, contact the support team to investigate. If you have trouble reaching us, see Get support in Access Financials.