I kept trying different fixes for remaining "Unverified" for weeks. When I finally explained the problem in the right way to the right ConvertKit support person, I just needed to do one simple step that had previously not been intuitive to me: Under Settings, and Domains and then Custom Domains, I had to click the "Remove this domain" button and then re-add it. Finally, the subdomain was verified.

Coach Laura
Teacher of English as a Second Language