CoCd &1 doc. &2 line &3 block removed as pmnt method of open itm is space

What causes this issue?

Open items with <ZK>Payment method</> = SPACE are dunned when:
They are RE items that are referenced with a rental contract that does
not have a dunning block.

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.