HTTP Version Not Supported

The HTTP version used in the request isn't supported.

505 HTTP Version Not Supported means the server refuses to speak the HTTP version the request used. Browsers negotiate versions automatically, so they almost never trigger it. When it does show up, it's usually a hand-crafted request with a bogus version line, or very old client software talking to a server that has dropped HTTP/1.0.

What 505 looks like on the wire

GET /index.html HTTP/3.5
Host: example.com

HTTP/1.1 505 HTTP Version Not Supported

About 5xx Server Error

The server failed to fulfill a valid request. The problem is on the server side, not the client.

How to fix 505 HTTP Version Not Supported

Let the client library negotiate - don't hardcode the HTTP version in hand-built requests.
Upgrade ancient clients or tools pinned to a version the server dropped.

Where it's defined

505 HTTP Version Not Supported is defined in RFC 9110 §15.6.6. MDN's 505 reference covers browser behavior and examples.