You may not enter a dunning lock for product type &1

What causes this issue?

There is no open item management for loans taken, loans with payment
request, or loans not managed on the basis of customer accounts.
Consequently the dunning function is not released and the dunning lock
can not be used.

System Response

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

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.