Please specify a valid account assignment

What causes this issue?

You have entered an account assignment category in the document item
for which an account assignment is mandatory.
All account assignment lines in the service specifications either have
been deleted or are inactive (e.g. account assignment lines of a
contingency item).

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?

Please enter an account assignment for the services or value limits.
The entry of an account assignment is unnecessary if you choose
"unknown account assignment" in the document item.

Error message extract from SAP system. Copyright SAP SE.