Hi KK2
Thanks for your post.
In this matter, you can only delete payments that haven't been reconciled or matched to a bank transaction. So if you want to delete a reconciled or matched payment you need to un-reconcile or un-match it first, using the Bank reconciliation or Bank Transaction page. You can check on this Help Article: Deleting a Customer Payment this includes more information.
If you need further assistance with this. Feel free to post again. We're happy to assist.
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Cheers,
Cel