This can happen if the Domain enforcement is on. Domain enforcement is enabled by default on all newly created Enterprise and Federated directories. All domains within the domain enforcement policy-enabled directories will have restricted authentication. Users can't create new accounts using restricted domains linked to policy-enabled directories. Domain enforcement blocks creation of new Adobe ID accounts with an organization-owned domain. For more details, visit: https://adobe.ly/3ZqStyi
Go to the Adobe Admin Console Settings to view your directories and their Domain enforcement statuses in the Adobe Admin Console. Then, select a directory with Domain enforcement status set to On to view the Domain enforcement settings section.
Note: Adobe strongly recommends that you keep domain enforcement ON for your directories. Turning domain enforcement on or off frequently may result in users facing disruptions while signing in.
Please let us know if you require further assistance.