Hello there,
The AISSEE Admit Card was released on 10th January 2021 and you can download it from the official website. Check the details which you input so that there's no error. The message shown to you is maybe because you have given a wrong input, or your internet connection is slow or some other issues. Please try to download your admit card again after sometime.

Procedure to Download AISSEE Admit Card 2021:
1. First open the official website aissee.nta.nic.in
2. At home page find Option to Download Admit Card AISSEE 2021 Exam.
3. Open that link and enter your Application Form Number and Date of birth & security pin.
4. Submit the correct details and download AISSEE 2021 admit card
5. Take the hard copy of admit card

If still you are unable to download your admit card, you can reach out the official authorities. You can write your problem and send it to their helpline number or email provided in the official website aissee.nta.nic.in.

For more information about the same, you can check the link given below:
https://school.careers360.com/articles/sainik-school-admit-card

Hope this information helps you.
Good luck!!