Yes, you can still fill out the NDA form even if you failed 12th class. The NDA eligibility criteria for educational qualification state that you must have passed a 10+2 exam or equivalent. There is no restriction on the number of attempts you took for 12th board exams.Since you have not passed 12th yet,  mention yourself as a "12th Appearing" student in the application form.

CBSE private students are valid for the NDA exam and most other competitive exams in India. There's no discrimination against private students when it comes to eligibility. The focus is on passing the qualifying exam (10+2 or equivalent) from a recognized board like CBSE.

While you can appear for the NDA with a 12th appearing status, it's crucial to pass your 12th board exams before the SSB interview stage. The SSB interview cannot be attended without a valid 12th pass certificate.Keep in mind the age limit for appearing in the NDA exam. You should be between 16.5 and 19.5 years old when you apply.

https://competition.careers360.com/download/ebooks/tips-and-tricks-crack-nda/na-exam

I hope it helps!