You must enter a reference for document type &

What causes this issue?

You have not entered a <LS>Reference</>. The Customizing system is set
so that a reference must be entered for document type &V1&.

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?

Check the document type. If it is correct, you must enter a
<LS>Reference</>.

Error message extract from SAP system. Copyright SAP SE.