Course enrollments may be restricted to only members of the university, or only specific groups within the university. If you haven't registered with that university yet, click Register or Sign Up on that university's website. If you don't see either of those options, click on Contact to reach an administrator who can help you set up an account.
If you are logged into your account and still getting an error, you should contact your University Administrator for assistance as the course may be restricted to specific users or groups.