Consumption only maintainable if period indicator entered

What causes this issue?

You want to enter historical consumption values on which the system
will base its initial forecast. Since the system manages consumption
values and forecast values on a period basis, you must specify the
periods in which this should happen.

How to fix this error?

Enter your period indicator. You can choose one using F4 and F2.

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

Error message extract from SAP system. Copyright SAP SE.