Hi Neehal,
For forecasting models, you need to first create the master forecast profile for your planning area, assign the history and forecast key figures, time horizon and also initialize the corresponding Forecasting methods i.e. Univariate/MLR and then you will be able to carry out forecasting in planning book.
You can refer to below links for more details -
Model Initialization - Forecasting - SAP Library
Forecast Profile - Forecasting - SAP Library
Hope it helps.
Regards,
Alok