Hii

NEET Eligiblity is that one must have qualified class 12th examination with physics, chemistry and biology/ biotechnology and score a minimum of 50% aggregate.
NEET has nothing to do with Maths so there will not be any problem if you have done your 6th subject from NIOS that too maths.
SO you need not to worry and focus on your exam preparations.
Incase you wish to get some help for your preparations, you can use our NEET Knockout course.
https://www.entrance360.com/knockout-neet-may/
It would provide you with the Doubt resolution options, a customized time table for your effective study plan, Study material with more than 5000 concepts with videos, previosu year papers and explained conceptual details.
You can also get advanced test series for your preparation and also an analysis tool to see how your performance is.

Hope this helps
Thanks!