Create a certificate for the server, so the communication between Cloudflare's proxy & your server is secure. At this moment, you need to create this certificate manually via Cloudflare Dashboard.
As mention (dash.cloudflare.com/xxxxxxx/domain.tld/ssl-tls/origin):
"Generate a free TLS certificate signed by Cloudflare to install on your origin server. Origin Certificates are only valid for encryption between Cloudflare and your origin server."