Hi aspirant, since you have completed your SSC exam which is most probably 10th standard in year 2017-18 so on behalf of this you surely can apply for not not IIIT but for JEE Mains in 2021 also. See one can get in IIIT if he/she have been given the JEE Mains and have eligible marks to posses the IIIT's and NIT's and for IIT one must qualify JEE Advance exam with a good rank.

Good Luck.