A 521 error occurs because the origin web server refused the connection from Cloudflare.
This means Cloudflare tried to connect to your server on port 80 or 443 but received a 'connection refused' error.
Could you share with us your server address?
Also, could you execute this in order to check if your server has the 80 or 443 port opened?
netstat -l SERVER-ADDRESS
Note: Take into account to change SERVER-ADDRESS to the ip direction of your server.