Billing class is inconsistent

What causes this issue?

Billing class &V1& of the gas procedure is different to the billing
class of the rate category in period &V2& - &V3&.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

STYLE S_DOCUS1
INCLUDE ISU_VERARB OBJECT DOKU ID TX
STYLE *

How to fix this error?

Adjust the billing classes.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.