Hello,
You can appear for JEE Mains exam 2021 if you satisfy the minimum eligibility criteria for JEE Mains.
If you have passed your class 12 in 2019 or 2020 or appear in 2021 , then you will be eligible for JEE Mains 2021.
For JEE Advanced, if you have appeared for JEE Advanced, if you have appeared for your class 12 in 2020 or will appear in 2021, then you will be eligible.
Also, you need to score minimum 75% (65% for ST, SC, OBC, PwD) in class 12 boards aggregate to be eligible for JEE Advanced and also to take admission into NITs IIITs and GFTIs through JEE Mains.

Please visit this link for more detailed information about the eligibility criteria for JEE Mains
JEE Mains eligibility criteria

Hope it helps

Joint Entrance Examination (Main)

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