Forecast only possible if you enter a period indicator

What causes this issue?

Period indicator has not been set in the material master.

System Response

As the system manages forecasting and consumption values on a period
basis, you have to select the period which the activity is to be based
on (for example, day, week, month or a period according to fiscal
variants)

How to fix this error?

Save your data and go back to the material master (without MRP area).
Enter the period inidcator in the appropriate view. You then repeat the
creation of the forecast data for the MRP area.

Error message extract from SAP system. Copyright SAP SE.