A 501 Not Implemented error is a webserver’s code for: "I don't know how to perform that kind of request method.".

How do I fix or troubleshoot a 501 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.

501 Error | Error-Message.Net

A 501 Not Implemented error indicates that the server does not support the functionality required to fulfill the request.

This is the appropriate response when the server does not recognize the request method (such as GET, POST, PUT, DELETE, HEAD, OPTIONS) and is not capable of supporting it for any resource.

Determining why

The client sent an experimental, obscure, or invalid HTTP method verb that the webserver software does not recognize.
The web server is an ancient software build lacking support for newer HTTP capabilities.
A proxy or security appliance between the client and server stripped or blocked the request verb.

Steps you could possibly take:

Check the request method - If you are using curl or an API testing tool, ensure you are using standard HTTP verbs in uppercase (e.g. GET or POST).
If you are the server administrator:
- Update your web server software (Apache, nginx, Caddy, IIS) to a modern supported release.
- Check server modules and routing rules to verify that required HTTP methods are not disabled.

Source: RFC 9110 Section 15.6.2 (HTTP Semantics)