PO item cannot be retrosp. converted into order-dependent 3rd-party item

What causes this issue?

This PO item does not depend on a
<GL:sales_and_distribution_document>sales document</> (that is, it does
not result from a sales document). It is not possible to reference a
purchase requisition that was created from within a sales document.

How to fix this error?

You can only reference this purchase requisition in a third-party order
item. In the PO item you are currently processing, you can only
reference purchase requisitions that have not been created from within
a sales document.

Error message extract from SAP system. Copyright SAP SE.