Hey!
For NDA exam, it is compulsory for students to have passed standard 12th with subjects of maths and physics. So, you being a commerce student may have had maths but not physics so hencewhy you won't be eligible for the exam.
For further information regarding the eligibility, check out the link provided below.
https://competition.careers360.com/articles/nda-eligibility-criteria
Application | Eligibility | Admit Card | Preparation Tips | Result | Answer Key | Cutoff
Regular exam updates, QnA, Predictors, College Applications & E-books now on your Mobile