Cloudflare cannot access the web server hosing the website.
Check that the server hosting the website is actually running.
Check that your firewall allows Cloudflare IP addresses to access it, and so forth.
Contact the hosting provider, that are providing the server you use to host your website.
The “Additional troubleshooting information” link, as mentioned on your error photograph, will provide useful hints, too:
If it isn’t your website (and server), there is nothing you can do from your end.