Hi Oluwafemi Otusanya, I want to help you with this question.
If I understand you correctly you started the validation process for a custom domain you ordered via Microsoft or via another domain provider, right?
I had the same problem and I solved the problem after adding a TXT record for the name "asuid" with the generated value in the DNS. After adding the new record, the validation was successful after around 1 hour.
If the reply was helpful, please don’t forget to upvote or accept it as an answer, thank you.