That’s a really interesting case. Your site does resolve for me, even though it looks like the whole DNS infrastructure serving your domain is broken, at least from the answers to the queries I send. Your site is served by ns1/ns2.imfellow.me. Which by themselves are served by ns138/ns139.availabledns.com. But when you ask ns138/ns139 for the addresses of ns1/ns2.imfellow.me - they do not exist! (getting NXDOMAIN).
If Cloudflare is receiving the same responses I do, then the error message you got from Cloudflare totally makes sense. The really interesting question is how the site still works for you. Maybe you’re using Cloudflare’s DNS (184.108.40.206) - which successfully returns an answer when querying for this domain (maybe not forever…). If so, that might mean that people not using 220.127.116.11 may also not be able to access your site. I tested Google’s public DNS at 18.104.22.168 and got a SERVFAIL as expected.
If all the above is correct, then the solution is simple: Switch your domain’s DNS service to a service that works, before onboarding to Cloudflare. If you need a free DNS service, you could try dns.he.net. Once you changed the NS servers in the registrar, try to add in Cloudflare again, and I assume it will work at this point.