there is a dark spot in my class 12 marksheet passport photo. Does it will create problem in future
If your photo is visible and if you have all the details correct with your official ID card as well as your 12th Registration certificate and 12th Admit card then there will be no problem about it. You don't have to worry about it. But if you feel that your face is not visible good then you can go for corrections, but in general it will not cause in problems.
For corrections you have to contact your school/junior college.
I hope this answer helps. All the very best for your future endeavors!