Issues with Error 521 on my site

While using https://tools.keycdn.com/curl
Below is the error message, hope someone can help?

HTTP/2503

cf-cache-status: MISS

cf-ray: 68f0d6abadb14db2-FRA

content-type: text/plain; charset=utf-8

date: Wed, 15 Sep 2021 09:30:50 GMT

display: staticcontent_sol, orig_site_sol

expect-ct: max-age=604800, report-uri=“https://report-uri.cloudflare.com/cdn-cgi/beacon/expect-ct

expires: Tue, 14 Sep 2021 09:30:50 GMT

response: 521

server: nginx

vary: Accept-Encoding

vary: Origin,Accept-Encoding

x-ezoic-cdn: Miss

x-middleton-display: staticcontent_sol, orig_site_sol

x-middleton-response: 521

x-origin-cache-control: no-store, no-cache

x-sol: orig