Just to confirm, you were able to resolve your issue. If so, the Microsoft Q&A community has a policy that the question author cannot accept their own answer. They can only accept answers by others, I'll repost your solution in case you'd like to Accept the answer.
Issue: Your domain could not be verified for an ACS resource even though you added the necessary TXT record.
Resolve: You updated the appropriate TXT record, and the domain was successfully verified.