Check the following: 

• Is the course expired or inactive?
• Is the domain expired or inactive?
• Has the user already been added to the course? Check the course settings 
• Has the user already been added as a domain user? Check the domain users.
• Is the domain a top-level domain and the user is in a subdomain? Check the domain setup settings. The user must is in a subdomain and the course on the top domain.

If none of these are the cause of the issue or you get an error message, contact Support for assistance.