Yes, you can appear for Jamia Entrance test after completing your 12th. However, please check the required qualification needed for appearing in JMI entrance test as the basic eligibility varies for different courses.
For more details , you can visit the link provided below:
If you are asking about the eligibility for the exam conducted Jamia Millia Islamia University for UG programs, then you need to specify for which course you want to apply so that I can help you.
Below I've attached a link which has information about all the courses offered by JMI.
Hope this helps