SAP Message AH663 - Key date is missing for month-based billing

Diagnosis
Installation &V1& contains rate category &V2& untilthe end of the billing period. The indicator interprettax changes on a monthly basis is set for this rate category.Therefore, a key date for billing on a monthlybasis must be specified in the meter reading unit of the installation.This is not the case.

System Response
The installation cannot be billed.