i am a 11 th class student and preparing for jee but during some issue i have failed in 11 th and now i don't want to do it again and also want to give jee in 2021. so now what can i do?? can i fill 12 th private form and after doing this will i eligible for jee ?? please answer

Hey there hope you're doing well.
Dear aspirant, No issues if you have not passed your class XI exams. Give it another try and you'll surely suceed. Yes you can parallely prepare for your JEE MAIN for 2021. Also , yes you can appear class XII privately. Even after that you would be eligible for JEE MAIN.  There is no age limit you would still have 3 attempts for JEE MAIN and 2 for JEE Advanced on and after year 2021 ,the board appearing year.
Thank you hope it helps you!