Rule &1 cannot be deleted

What causes this issue?

Rule &V1& cannot be deleted because it is still used in at least one
accounting transaction.

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?

You can only delete rule &V1& if it is no longer used in any accounting
transactions. Therefore, you should check the transactions in which
rule &V1& is being used.
Note: You cannot delete rules in the SAP name range (01 to 89).

Error message extract from SAP system. Copyright SAP SE.