This is Silviu, Technical Support Engineer (Tier ll) within Okta. The "domain not being able to be registered" when trying to set up an infrastructure with an Okta AD Agent sounds like a Firewall issue. If I am to pinpoint I would say you have to whitelist some domain name services. Still, we do need a higher level of detail to properly troubleshoot such an issue. Please open up a case with us and we'll surely get a better understanding of the situation that way. You can even get with us in a remote screen-sharing session.