A 426 Upgrade Required error is a webserver’s code for: "You need to upgrade to a newer protocol (like TLS or HTTP/2) before I'll let you in.".

How do I fix or troubleshoot a 426 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.

426 Error | Error-Message.Net

A 426 Upgrade Required error indicates that the server refuses to perform the request using the current protocol, but might be willing to do so after the client upgrades to a different protocol.

The server transmits an Upgrade header field in the 426 response detailing the acceptable protocol versions (for example, demanding an upgrade from HTTP/1.1 to TLS/1.3 or to WebSockets).

Determining why

The client attempted an unencrypted cleartext connection to an endpoint requiring modern encryption.
A client initiated a WebSocket handshake using an obsolete or incompatible transport protocol.

Steps you could possibly take:

Update your web browser or client software - Ensure you are using a modern browser that supports HTTP/2, HTTP/3, and modern TLS standards.
Use HTTPS - Change the web address prefix from http:// to https://.
If you are a developer - Check the Upgrade header sent in the server’s 426 response to see which protocol the endpoint requires.

Source: RFC 9110 Section 15.5.22 (HTTP Semantics)