I passed 12 board in 2019 but not appear in any jee exam this year. I appeared for jee main first time in 2020.what i can give jee advanced 2021

Dear student,

In jee mains the maximum number of attempts allowed are three. You first attempt is counted as over in the year that you have passed your class 12th that is in 2019. And it doesn't matter that got have given jee mains exam in 2019 or not, still your first attempt is counted as over.
After that you can maximum give two attempts in next consecutive years.
So, in 2020 your second attempt of JEE mains is over and jee mains 2021 will be your last attempt.

So if you have given jee mains this year 2020 first time then your second attempt is counted as over. And next year - that will be your third and last attempt of jee mains and after that you can not give jee mains.

Now in JEE advanced you can give maximum of two attempts and your first attempt is counted as over in your class 12th passing year.
That means in 2019, your first attempt of JEE advanced is counted over and it doesn't matter whether you have given JEE advanced exam or JEE mains exam or not.
Your first attempt is over now and this 2020 year was your last attempt for jee advanced and hence in 2021 you cannot give Jee advanced exam.