Hi danielpwalker,
I have checked the DNS record on the primary domain you provided and found that there is a record which uses for verifying ownership has been added.
I’d like to suggest you check if you have registered any other Office 365 plans which is using (verified) the domain you are going to add in the current account. If there is one account has already verified the domain, I suggest you remove the domain in that account and then add it to the current account.
Thanks,
Kent Gu
