Dear Student,

Every student get 6 chances of JEE Mains and 2 chances of JEE Advanced. Now as per your question you had given your 1st JEE Mains in 2019, so clearly your 1st chance of JEE Advance goes over. And now in 2020 you give your 2nd JEE Mains and JEE Advance also. So clearly, all of your JEE Advance chances are over now. So you can't give your JEE Advance further in 2021.

But if you are not interested in Mechanical branch, then surely you can give JEE Mains one more time in 2021. And this time do your best. And if you get a very good score this time , then you can choose some top NIT and also the branch of your choice.

Hope this helps.