• Do you have the necessary permissions? You can only add/create a domain if you are a Domain or Enterprise Admin (some other roles may be given access in special cases).
• Have you filled in all the required fields?
• Are you trying to create the domain in the right location? and where you created the domain. There is a default domain/parent domain so be careful in adding a new domain to a non-parent domain. This will make the domain a sub domain.

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