Hello,

The eligibility for polytechnic courses is class 10th or equivalent exam . Therefore if you have passed 12th and interested in polytechnic course you can go ahead and join. There is no need to wait for completing your graduation.

As polytechnic is a vocational course a regular graduation will do little to enhance its relevance. However, if you are interested in pursuing graduation also you can opt for distance learning graduation simultaneously  or enroll in it once you are done with polytechnic course.

Hope it helps.