Please share details on which exam CBSE/ NEET/ SSC/ State Board, etc. admit card you want to download. Usually, it is not possible to download the admit card using only the roll number. To download an admit card, you usually need one of the details like Registration number / Application number, Date of Birth, Password, and Registered mobile number for OTP verification.

Meanwhile, check these links out:

NEET Admit Card Link
GATE Admit Card Link