Hello aspirant

If you fail in physics xii board  , and get good marks  in other subjects . Getting good marks in JEE Main. If you pass  Your failed subject  then no need to worry. But you must qualify for the aggregate percentage in board. That will be needed in nit or IIT. After that you on the basis of JEE main you will get admission in your desired stream.

It won't affect your Jee rank but it will affect you

rank in merit list in  institutes ( where you will apply)  at the time of admission.  As at the time of admission all factors are considered  . After jee main score 12 th marks are considered . First attempt  or second attempt   , this is also a factor.