Proxy error: UnprocessableEntity, Failed to get installation and oauth information for app

I have been guided here by Zendesk support, I am receiving the error Proxy error: {:code=>"UnprocessableEntity", :status=>"422", :title=>"Unprocessable Entity", :message=>"Failed to get installation and oauth information for app."} when I attempt to create a request to a 3rd party server, I have followed your extensive guide but still recieving the above.