No forecast possible with forecast model &

What causes this issue?

You wanted to do one of the following:
Enter <DS:GLOS.forecast_value>forecast values</>
Execute a <DS:GLOS.forecast>forecast</>
However, the <DS:GLOS.forecast_model>forecast model</> specified does
not allow forecast values.

System Response

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

How to fix this error?

If you want to maintain forecast values or execute a forecast and use
the results in a <DS:GLOS.planning_run>planning run</>, you must choose
a different forecast model.

Error message extract from SAP system. Copyright SAP SE.