Hi

First of all there is no percentage barrier of 75% or any other minimum percentage requirement to be eligible for JEE Mains.

As ,
Eligibility criteria says that to be eligible  for JEE mains 2021 for BE or Btech you just need to 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.

But yes, there is percentage barrier for
to be eligible for NITs, IIITs, GFTIs , jee advanced( for iits) as to be eligible for these one needs at least 75% marks in aggregate in class 12 if belong to general or obc category or at least 65% marks if belongs to SC/ST or must be in the top 20 percentile in class 12 examination. But for this year i. e 2021 , this percentage barrier has been lifted off for only IIT, and there's no minimum percentage requirement to be eligible for IITs

Also,
There is a small change in JEE Mains examination pattern , in your jee mains 2021 there will be 30 questions in each subject ,20 questions will be MCQs and rest 10 questions will be questions with numerical value as the answer , out of which only 5 questions are  to be attempted.

Now, that
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  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.

For detailed information go through

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

Thankyou