Refer to https://learn.microsoft.com/en-us/answers/questions/1614401/cannot-add-custom-domain-microsoft-entra
Find your domain registrar If you have received no response, you need create a ticket with Microsoft support team with the tenant ID by inform them stating as that you are an owner of the custom domain and unable to verify custom domain for organization because the domain is verified in another tenant.
Once the ticket is created with support team you will have to work with our data protection team and validate your identity against your tenant for security purpose post that this team will help you on your issue.
Hope this helps. Do let us know if you any further queries.
Reference: https://learn.microsoft.com/en-us/entra/identity/users/domains-admin-takeover
hth
Marcin