Check the following: 

• If you are in a top-domain setup, you must be in the subdomain and the course must be in the top domain. 
• Check that you have entered all the required fields. Personal information such as first and last name, username, password, verify password are all required.
• Check that you have the necessary permissions to create a user. A standard user cannot add users to the system.

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