A previously used distribution rule already exists

What causes this issue?

When you try to change the distribution rules, rules were found that
have already been used in a settlement invoice.

System Response

Existing distribution rules cannot be deleted if they have already been
used. Un-used distribution rules can be deleted.
No additional distribution rules can be generated, as outlined in the
strategy in the settlement profile.

Error message extract from SAP system. Copyright SAP SE.