There is no such rule that only after scoring 750 you will be eligible for joint entrance examination (JEE) main. To be eligible to give joint entrance examination (JEE) main a candidate needs to qualify 10+2 level or Candidate appearing in class 12 or equivalent qualifying exam in 2020 can also apply for JEE Main. Please click the below link to know more.
The basic eligibility criteria for JEE Mains is that the candidate must obtain 75 percent marks in class 12 or equivalent exam or they must be in top 20 percentile in their class 12th exams. For SC/ST,the eligibility is to obtain a minimum of 65 percent marks in class 12 or equivalent.
So, by taking this to account, you are eligible.
For detailed information follow the link given below :
Hope this helps.