Domain was added to a shadow tenant – this normally happens you took ownership of the domain that was added to an abandoned tenant
The domain was used for free trial such as PowerBI, Windows Store, and Microsoft flow that by extension will create a temporary tenant for the domain
You are getting this error when you add the domain
<DomainName> has already been verified for your account, or for another Microsoft Online Services account.
Can’t verify the domain <DomainName> was already added to a different Office 365 account domain.onmicrosoft.com. Sign in to that account as an admin, and remove domain <DomainName>. Then come back here and try adding <DomainName> to this account again