Hello aspirant, hope you are doing good.

the main criteria for applying in JEE Mains is that you need to have been qualified class 12. So, the fact that you have successfully passed class 12 in 2019 itself, you can definitely apply for the exams in 2020 and even 2021. You obviously have to fill up the form for JEE Mains first,because only if you pass JEE Mains, you will then be eligible to sit for JEE Advance.

Hope this helps you. Good Luck!!