Hello there,

Let me tell you that you can give the JEE Mains exam only 3 times in your life. The one time when you were in 12th and the rest two time are consecutive two years after passing your 12th i.e. 2019 and 2020. Now as you have passed 12th in 2018, so let me clear you that, you can't give JEE Mains 2021. All of your chances are over now. To know more about the jee mains eligibility, visit the link.

https://www.google.com/amp/s/engineering.careers360.com/articles/jee-main-eligibility-criteria/amp

Hope it helps.