Key date is missing for month-based billing

What causes this issue?

Installation &V1& contains <DS:DE.TARIFTYP>rate category</> &V2& until
the end of the billing period. The indicator <DS:DE.MWSTMBEZ>interpret
tax changes on a monthly basis</> is set for this rate category.
Therefore, a <DS:DE.E_STICHTAG>key date</> for billing on a monthly
basis must be specified in the meter reading unit of the installation.
This is not the case.

System Response

The installation cannot be billed.

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.