Diagnosis
Billing document &v1& has end of billing period &v2&.
The shortest retention period in days results in the key date &v3&.This retention period is calculated from the retention periods for thearchiving document ISU_BILL (IS-U archiving: billing document) formanual/normal billings, final billings,and backbilling/period-endbillings, using system date &v4&.
However, the end of the billing period is before this key date. In thiscase, the billing document can no longer be reversed since it is readyto be archived.

Procedure
If you still want to reverse the billing document, create a manualcredit memo or backbilling.