The eligibility criteria of NEET suggests that the student needs to qualify all the paper in 10+2 and he/she must have atleast 50% marks in the aggregate of physics, chemistry & Biology (For OBC-45%, SC/ST-40%). So if you fail in the 10+2 examination, then you can sit for the examination of NEET but you won't be considered eligible for the admission to medical courses through the NEET score.

For more information regarding NEET consider the link below:

https://medicine.careers360.com/articles/neet-eligibility-criteria

I hope this helps. Wish you a great eve!