The most common reason for not being able to log in to ILL is using the wrong username.
Your username is the first part of your TAMU-CC email address, the part before the @.
For example, for the user Fozzie Bear, the email address would be fbear@islander.tamucc.edu, but the username would be fbear.
The password is the same as the one you use for your Islander email, library databases, etc.