Error – ‘One or more of the transactions selected is currently being reconciled using the Automated Bank Reconciliation functionality.’

This means that a transaction is matched or partially matched in a statement in the Auto Bank Reconciliation function.
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To resolve this, you will need to go into the Auto Bank Reconciliation function (aka ABR) and go through each statement to locate the transactions and then remove/clear the match or delete the statement if it is no longer needed.
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You are looking for lines in a statement in the left-hand panel marked with a status ‘C’ (fully matched) or ‘P’ (partially matched).