Attempting to delete a domain on the Administration Console;

 However the following error appears:

This usually means the domain is still tied to something else, for example, a realm, a policy, a role, etc

The first thing to do is check the obvious places on the Admin Console

Authentication > Realms

Check under the domains column...

If you see it in there (as per the example above), edit the realm and remove it.

Administration>Policies

Check all domain/group/user policies that you may have created or edited recently

Administration>Roles

Check if any roles are tied to a domain