It can be an error. Please meet your institution teacher and clear this issue. If it is competition exam then please contact them or email them.

There are chances that admit card has not been released yet, as there is no official announcement, yet if they are available and you are not able to download it, have patience sometime the server gets down and servicem might not be avaiilable for some time. Don't worry you will get your admit card before date of exam i.e. 27th May 2018.