POST http://127.0.0.1:5500/auth 405 (Method Not Allowed) / Uncaught (in promise) Error: No token defined

I am trying to build a chatbox using JS, Scaledrone and GO.
However, using their tutorial I have encoutered many errors during coding.

Most of them I have solved myself but this one gives me a headache:

After compiling the code, I get this response in console:

POST http://127.0.0.1:5500/auth 405 (Method Not Allowed)
and underneath that this one: Uncaught (in promise) Error: No token defined

It points at one part of my code and it is this one:

const formData = new FormData();
  formData.append("clientID", drone.clientId);
  fetch("/auth", {
    method: "POST",
    body: formData,
  })
    .then((res) => res.text())
    .then((jwt) => drone.authenticate(jwt));
});

It points exactly at

fetch("auth",

I managed to open the chat box, it gives me option to write and send message. It does not assign me name, it does not log my message into chat box.

using sources from:
github.com/golang-jwt/jwt/v5
github.com/gorilla/mux

I have tried different things, from changing the /auth information to googling all possible solutions but none gave me the answer.

There are simpler options to build a chatbox using vannilla js but I would like to do it this way as it looks more cool.

I have updated all the GO files, downloaded GO manually, and also updated github.com/golang-jwt/jwt/v5 .