I have used the Number of Fourier terms as 25 which could be correct for Weekly seasonality because it is less than half of 52 weeks. And I have arrived at the solution for this problem. The problem i mixed upto functions of two packages fable and forecast. When I used the correct model function of the fable package it had run correctly. Thank you so much for the comment. I will close this thread. However I'm stuck at calculating the accuracy of the forecast object using fable- Error in accuracy calc. using Fable. If you could guide me on this, would be really helpful