Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello
In Microsoft 365, a domain can only be associated with one tenant at a time. If the domain is currently connected to another Microsoft 365 tenant, it must first be removed from that tenant before it can be added to your current Microsoft 365 subscription.
To help identify where the domain is currently registered within Microsoft 365, could you please confirm:
1/ What is the domain name you're trying to add?
2/ Do you have access to any previous Microsoft 365 tenant that may have used this domain?
3/ What is the exact error message you receive when adding the domain?
4/ Is this a brand-new domain purchase, or has the domain been used previously for Microsoft 365, Exchange Online, Teams, or another Microsoft service?
If you no longer have access to the Microsoft 365 tenant that currently owns the domain, Microsoft Support may be able to assist with releasing the domain after verifying your ownership. Please provide the domain name and the exact error message (or a screenshot) you receive when adding the domain.
You can contact Microsoft Support from the Microsoft 365 Admin Center by going to Help & support and creating a support request
https://admin.microsoft.com/#/support/requests
Please let me know if I can do anything more for you.
Kind regards,