Hi

See the basic eligibility mentions that if you have either studied or studying MBBS you are eligible to apply. But in your case, where you are enrolled in some other medical programme you need to apply under 'Special Permission' category. The officials will evaluate your profile and will revert with your application acceptance status. 

The officials have mentioned this on their website under FAQ section 

If you aren’t planning to apply to a health professions school or if you’re a currently enrolled medical student (other than a MBBS degree program), you must obtain "special permission" to register for the MCAT exam.

To request special permission, please send an email to mcat@aamc.org, stating the reason(s) you wish to take the exam. The MCAT exam office will attempt to review and respond to your request within five business days, although during certain times we may be delayed in our ability to reply within this time frame. We therefore ask that you be mindful of registration deadlines, because staff cannot extend closing dates for any reason."

Check this link: https://students-residents.aamc.org/applying-medical-school/faq/mcat-faqs/ 

All the best