A 510 Not Extended error is a webserver’s code for: "The server requires additional protocol extensions before it will fulfill your request.".

How do I fix or troubleshoot a 510 Error?

Common steps include reloading the page in your browser, checking for typos in the URL, verifying server status, clearing browser cache or cookies, and contacting the website owner or server administrator if the error persists.

510 Error | Error-Message.Net

A 510 Not Extended error is an HTTP status code defined in the experimental HTTP Extension Framework (RFC 2774).

It indicates that the policy for accessing the resource has not been met, and that the server requires the client to send further protocol extension information before access will be granted.

Determining why

The server expects the client to support specific mandatory extension headers that were omitted from the request.

Steps you could possibly take:

If you are a visitor - This is an extremely rare status code. Contact the website owner to report the issue.
If you are a developer:
- Inspect the Mandatory and Opt extension headers required by the target endpoint.
- Update your API client to include the required extension declarations.

Source: RFC 2774 Section 7 (An HTTP Extension Framework)