Oh I see, you are trying to create users, not list them. In that case ignore my previous post 
If you visit <discourse-url>/users.json in your browser, it will return that “not found” error. That is to be expected. Sending a POST request to that URL, along with all the necessary parameters, should work fine. The docs for the endpoint are here: Discourse API Docs
Can you share the exact code you are using?