Hello Ashish !

Though there is no official release of cutoff marks and rank by the IISERs but it is very unlikely i.e impossible to bag a seat in any of the IISERs with 21 marks in IAT with any kind of reservation.

It is generalised that about 75+(with reservation) in IAT may get you a seat in the newest IISERs. But 21 marks is too low....

Please note that that the board cutoff for every board is relaxed upto 60% due to the COVID-19 crisis. And you just have to cross the cutoff, it doesn't play such a big role for getting you in the merit list, only your marks in IAT does(along with your reservation)

There is no need to be disheartened...you can try again and excel in the exam again. When there is will, there is way !

All the best !