The eligibility criteria for NEET is that the candidate must be older than 17 years. The minimum qualifying marks for NEET is that in 10+2 the candidate must have (marks eligibility corresponding to the category provided below):

• UR - 50%,

• OBC/SC/ST - 40%,

• PWD - 45%

Another eligibility is that you must have Physics, Chemistry and Biology in 10+2 with English as an subject.

Thus it doesn't matter you have PCB in first attempt or PCMB in second attempt. What matters is that you should satisfy this eligibility condition.

To know more about this visit the link provided below:

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

I hope this answer helps.