Hello Aspirant

If your syllabus for PCM additional was the same as the students who actually study PCM as their main subject, then yes, you are eligible for JEE Mains. But make sure that you have that mentioned in your marks card. The eligibility criteria for JEE Mains is that you should have a 55% aggregate in your 12th year with PCM as your subject. For JEE Advance, you need to have 75% and above percentage in 12th.  If you fulfill this criteria, then you can sit for the exam.

Hope this helps