That’s not a Cloudflare error. That’s coming straight from your server. I advise against using Authenticated Origin Pulls and instead use a server firewall to block against non-Cloudflare access. Digital Ocean has a nice firewall configuration setup that makes this easy.
It seems your server requires client certificate authentication. Either you have configured Authenticated Origin Pulls on your server side or you actually require authentication in that way.
In the latter case you couldnt really do this via Cloudflare (only Enterprise plans support that). If it is the former case, you should either make sure you Cloudflare account has that feature enabled and your server requires the right certificate or you disable it on your server altogether.
In short, you need to go through your server logs and configuration and check what is going on.