HELLO VIKAS

UPSC(UNION PUBLIC SERVICE EXAMINATION) conducts THE NDA(NATIONAL DEFENCE ACADEMY ) exams.  THis exam can get you into Navy,Air force,etc

UPSC has constructed set of rules for the eligibility criteria to give the exam.

1. the candidates should be unmarried.

2. The age limit starts from 16.5 years to 19.5 years.

3. Minimum qualification to apply for NDA is 10+2(physics and maths for air force and navy),10+2(army)

As you can see you clearly fit into the first two but you should be passes or appearing for 12th examination can apply for NDA and write the exam.

That is the reason you are not allowed to write the exam. For detailed information of the eligibility criteria you should visit the link given

https://competition.careers360.com/articles/nda-eligibility-criteria

HOPE IT HELPS!!