This forecast model allows no further field in the profile

What causes this issue?

You have chosen a forecast model for this profile that does not
accommodate any forecast calculation for the material or you are using
an external forecasting system. Therefore, no further fields, relevant
for the forecast, can be recorded in this profile.

How to fix this error?

Delete the fields that are relevant for the forecast or choose another
forecast model.

Error message extract from SAP system. Copyright SAP SE.