Hi@ Iniobong Nkanga
Thank you for connecting with us in Microsoft community.
According to your error message, you will have to remove custom domain from the Microsoft 365 Business organization tenant where it has been previously registered. Error message when you try to verify a domain in Office 365 - Microsoft Support
Furthermore, you will have to contact Microsoft support team so that they can help you from their side. They may involve specific support team from their backend resources. Here is official document to get Online Support: Get support - Microsoft 365 admin | Microsoft Learn
I will be greatly appreciated your valuable cooperation.
Have a great day.