When registering a new domain on Cloudflare, you need to keep two things in mind:

• They don’t allow their domains to have non Cloudflare-branded nameservers. Which means that while the domain lives on Cloudflare, its DNS records will also have to stay.
• Even newly registered domains have SSL/TLS configured to Full, so if you’re starting out with a new server, certbot will fail because HTTP traffic is not allowed.