Faizal if you are not able to clear JEE Advance examination then yes you can definitely sit in JoSAA 2018 counselling for getting admission into NITs, IIITs through your JEE Mains score. You should keep one thing in mind that the counselling of IITs and NITs both are done together so don't get confused and target for good NITs in your priority order according to your JEE main score so that you don't miss the good college. Along with this as I said earlier you can also apply for IIITs which are also good colleges for pursuing B.Tech. And along with this using your JEE Mains core you can also apply into private colleges separately for admission.

Feel free to ask if you have any more queries. All the best!!

Definitely you I'll be eligible to apply the admission in any NIT..