Hello @AmitSri,
the web applications (Environment Manager, VA, etc) all work with the authentication as set as web authentication: LDAP, SAML, etc. in your case, it is connected to LDAP/AD.
However, CAS sessions connect through web (SASStudio), but also it starts a process (the actual CAS one) in the host itself. That is why you need host authentication as well. And the users and passwords should match.
PAM is your friend here, and you can delegate authentications to it. So you can create users locally, and maintain the users and passwords, or you just join the machine/s to LDAP realm to allow users to log locally to the server (through SAS Studio) and the server should create the linux home folder for the user as well.