Ideally, you need to get a certificate on the origin and make sure the site is secure and then add it to cloudflare. Troubleshoot the error CERT_AUTHORITY_INVALID with the above tip for the steps & options. Let us know how it’s going.
Can you click that “Advanced” button and post a screenshot of the result? It should show you the cert that’s being rejected. My guess is that you’re bypassing Cloudflare due to some DNS setting you have.
What’s your SSL/TLS setting? It should be Full (Strict) as a server should have a proper certificate for your site, but I’m afraid that’s not the case with this domain.
For whatever reason, you’re going direct to the server. Either you have a local DNS issue, or you have a local hosts file with your server’s IP address. You’d need to get rid of that local hosts file entry.