Leave Reverse DocType empty or use doc type with same bus trans as orig.

What causes this issue?

In the <LS>Reverse DocumentType </>field, you entered a reversal
document type that differs from the document type given in the
<LS>Document Type </>field. The business transaction that is assigned to
document type &V1& differs from the business transaction that is
assigned to document type &V2&.

System Response

If you save your current changes, you might run into errors in the
reversal process. If you want to enter a reversal document type that
differs from the original document type, you should ensure that the
assigned business transactions are identical.

How to fix this error?

To avoid the risk of errors in the reversal process, you should choose
one of the following actions:
In the <LS>Document Types </>details view, leave the <LS>Reverse
DocumentType</> field empty. Then, by default, the system posts the
reversal document using the same document type as the original document
and thus ensures a correct reversal.
Enter a reversal document type in the <LS>Reverse DocumentType </>field
that has the same business transaction assigned to it as the document
type given in the <LS>Document Type</> field.

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

Error message extract from SAP system. Copyright SAP SE.