Diagnosis
Billing document &V1& originates from the legacy system (via migration)and can be reversed in billing only - not in invoicing.
To execute this reversal, you must use the AdjustmentReversal of Billing Documents function.

Procedure
Check whether it is really necessary to reverse legacy document &V1&,which originated from the migration.
Alternatively, you can use the Manual Billing functionto create a credit memo/backbilling.