i do my 12 in 2019 and i got 63% and i repeat my 12 same board in 2021 for increasing 12th marks i am eligible for jee mains or not

Hi

Yes, you are eligible for JEE mains 2021.

This is because,

One can give JEE Mains entrance  examination  for a maximum of  3 consecutive years  starting from the year of passing 12th board examination.

So if you passed your 12th board examination in 2019 you were eligible for JEE mains 2019 , 2020 and you are eligible for JEE mains 2021 .

So, yes you are eligible for jee mains 2021.

As

Eligibility criteria says that to be eligible  for JEE mains 2021 for BE or Btech you should be either appearing for 12th board exam or equivalent  in 2021 or should have passed it in 2020 or 2019 with Physics,  mathematics and anyone of Chemistry or Biology or Biotechnology or Technical Vocational Subject. Also there is no percentage criteria to be eligible for JEE Mains

For detailed information go through

https://engineering.careers360.com/articles/jee-main-eligibility-criteria

Limited time is left for jee mains 2021 , so to prepare well for your jee examination, go through chapters first and your focus should be on grasping concepts , practising questions and then revising them, go through mock test paper sample papers and previous year question papers as it will let you know your preparation level and and weak areas so that you can work on them and perform well in your examination.

Thankyou