Hello Ashish !

Though there is no official release of cutoff marks or rank by the IISERs, but it is generalised that at least 70+ marks is required for reserved seats in the newest IISERs.

It is very unlikely for you to get any IISER with 21 marks in IAT. Please note that your board % doesn't play a great role for getting you in the merit list because you just have to cross the cutoff for SCB to appear IAT.(i.e 60% for all boards this year)

There is another chance for you in the next year.

All the best!