Hi,

NEET eligibility criteria says that you must have passed Physics, Chemistry, Biology/Biotechnology, English with a 50% aggregate marks in class 12 examination from a recognised board or equivalent. As you have reappear in theory in biology subject, you cannot appear for the NEET until or unless you have qualified the biology examination. Indeed, you need to get 50% aggregate marks but you should be pass. So, you can fill out the reappear form, pass the examination and can give your NEET examination in 2021 itself.