Manual credit memo/backbilling &1 cannot be reversed

What causes this issue?

A manual credit memo/backbilling with no link to invoicing/accounts
receivable and/or payable cannot be reversed with the billing reversal
function.

System Response

Processing was canceled

How to fix this error?

You may need to delete the document using a function key in Transaction
EA16.

Error message extract from SAP system. Copyright SAP SE.