This is a standard message warning that appears when clicking Reconcile Now. It's to inform users that once transactions are reconciled, you cannot edit them.
Select Yes to the message to post transactions before reconciling or No to reconcile now.
Note: Posting or not posting will not effect the General Ledger balance showing within the Reconciliation window. Any items that are set to Not Yet Posted when the Reconciliation is taking place are included in the balance reflected for the General Ledger in the Reconciliation.