Error 521 means your server rejected the connection request from Cloudflare. Make sure you have entered the correct IP address in the Cloudflare DNS, your server’s firewall is allowing Cloudflare IP addresses, your web server is up and accepting requests on port 80/443 as required.
You can debug by setting the DNS record to “DNS only” to bypass Cloudflare, check the issue and fix it, then re-enabled Cloudflare by setting the records back to “Proxied”.