No reversal possible. Reverse value must be reset

What causes this issue?

You are attempting
IF &V3& = ' '.
to post a parked credit memo.
ELSE.
to reverse an invoice.
ENDIF.
In the item for PO &V1&, item &V2& the quantity is the same as the
total quantity invoiced, but the value is not the same as the total
value invoiced. This is not allowed.
Example:
,,Quantity,,Value
Invoice 1,,80,,800
Invoice 2,,20,,200
Credit memo,,80-,,850-
No credit memo can be posted with a quantity 20 and value 200.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

System Response

How to fix this error?

Display the PO history for PO &V1&, item
&V2&. You may have to reverse credit memos that were wrongly posted.
Otherwise, you may have to
IF &V3& = ' '.
change the parked credit memo.
ELSE.
manually enter a credit memo rather than reversing the invoice.
ENDIF.
You must enter the total value invoiced as the item amount and post
the difference created to a separate account.

Error message extract from SAP system. Copyright SAP SE.