Hi Atri,
Did you create any forecast model for your planning area? I assume there is no one available given that you are not able to select any from the excel drop down list <Forecast Model>.
Please follow these steps in order to define one:
Start Fiori Launchpad -> Manage Forecast Methods -> click Create button from the bottom right corner
After you select your planning area, you will have to input similar parameters as in IBP 4.0: historical and future periods, input and target KF, algorithm, coefficients.
If you would require further guidance in creating a forecast model, kindly check the application help:
Manage Forecast Models - SAP Integrated Business Planning - SAP Library
Regards
Alecsandra