Are you the owner of https://www.flexclip.com/? If so, then I would recommend you follow this guide:
The 502/504 errors are caused by a problem connecting to an upstream server - meaning your server is trying to initiate a process and this fails to work as expected or times out. In most cases of 502 / 504 errors, back-end servers are not communicating correctly. When this happens, you will see color page with Cloudflare branding and the Error 502 Bad Gateway or Error 504 Gateway Timeout. Review this Community Tip for fixing 502 or 504 gateway errors.
If you’re not the owner of said domain, you will need to report this to them. This is an issue with their configuration and there is nothing Cloudflare can do here unfortunately.