If you have a back (compartment) in one subject in class 12, you can still appear for the NEET exam. But you can take part in NEET counselling only after you clear that subject and get your full 12th pass certificate.

To be eligible for NEET counselling, you must pass class 12 with subjects like Physics, Chemistry, Biology or Biotechnology, and English. Until your result is complete and you are officially passed in all required subjects, you cannot participate in counselling.

So, you can give the NEET exam, but you can join the counselling only after clearing your back paper.

Website for more information:
https://neet.nta.nic.in
https://nta.ac.in
https://mcc.nic.in