Document &1 item &2 is marked as completed

What causes this issue?

Item &V2& in document &V1&, to which the document you are processing
refers, is flagged as complete.

System Response

If the Complete checkbox has been selected for a line item, the system
does not permit any reference to this line item if this would lead to an
increase in the reduction amount in the line item.
If you want to make changes to a document that refers to a line item
flagged as complete, the system does not permit an increase of the
amount.

How to fix this error?

The completion indicator can be reset if required.

Error message extract from SAP system. Copyright SAP SE.