Same problem happened with me but I don't have solution 

What should I do 

Hello,
Please first check your internet connection.I suggest you to wait for an hour or two and if the problem persists then you need to contact the respective authorities for more clarification on this problem.
You can take guidance from the below website to fill NATA application form:
https://engineering.careers360.com/articles/nata-application-form
Good luck.

National Aptitude Test in Architecture

Application | Eligibility | Exam Pattern | Admit Card | Answer Key | Result | Cutoff | Accepting Colleges

Get Updates Brochure