Receiving 404 - The requested URL or resource could not be found


I have been trying to use the REST API, but certain endpoints consistently return a 404 with the message “The requested URL or resource could not be found.” The particular endpoint that is returning this so far is the “Get a single user by SSO external ID” endpoint, which looks like this:


I have provided the Api-Key and Api-Username headers and followed everything else asked in the set up guide, but I still keep receiving this response. Is there anything that I am overlooking. I would figure if my API Key were bad it would actually say that instead of a generic “Not Found” message, so I am unsure what to do at this point.