@kkalam can you please provide a few more details about the situations. Do you mean the user is told the username or password is include at the credentials prompt or do they successfully access the SAS Studio application but their session which executes their SAS code gives some error?
In a default deployment of SAS 9 based platform which includes the SAS Studio web applications user authentication requests are performed by the SAS Metadata Server. What is in the SAS Metadata Server log at the time the user attempt to authenticate? Can any user login? What happens if that user attempt to login with another SAS client like SAS Enterprise Guide?