Dear student,

If your class 12th marks is 50%, then you can do engineering, but by giving other exams like wbjee, etc.

Also for giving jee mains you can give even in 50% aggregate and can take admission in some state engineering colleges, but if you want to take admission in nits or iiits or gftis, then, your class 12th percentage must be above 75% in aggregate.
And if you don't have then you have to sit for an improvement test and then you can give jee mains of your percentage is above 75%.

Otherwise you can do engineering but by giving some other state exams and scoring a good marks in their entrance exams.

Also check out the following link to know more about jee mains eligibility criteria :-
https://engineering.careers360.com/articles/jee-main-eligibility-criteria/amp