Is it possible become a doctor after completing graduation in commerce?

Yes, you can get an authentic doctorate degree by completing your Ph.D in a specific subject, but you can't join an OPD and don't have permission to observe patients.

For this you must have PCB in 12th.

If you had PCB in 12th and if you have completed 25 years of age and if you haven't given the exam for more than 3 times then yes, you can become a doctor.