Error: Grant credentials are invalid

This is probably happening due to what you are sending in with the authorization call. Check here for more information. With App Access Only, the subject type should be user not enterprise. 

Alex, Box Developer Advocate

Thank you for the reply. The document you shared is the one I am using. I am getting the UserID from the General Settings tab and the OAuth 2.0 Credentials from the Configuration tab. Here is what I am attempting using .net box sdk:

var boxConfig = new BoxConfigBuilder("6hvihoiygmj1r00bs8gclv8f4jwhiqem", "<secret>").Build();
            var boxCCG = new BoxCCGAuth(boxConfig);
            Task<string> response = boxCCG.UserTokenAsync("17293138289"); //valid for 60 minutes so should be cached and re-used
            while (!response.IsCompleted)
            {
                //still waiting;
            }
            objBoxClient = boxCCG.UserClient(response.Result.ToString(), "17293138289");
            objBoxClient.Auth.SessionAuthenticated += delegate (object o, SessionAuthenticatedEventArgs e)

I have also tried using Rest Sharp like this:

RestRequest objRestRequest = new RestRequest("oauth2/token", Method.Post);
            objRestRequest.AddHeader("Content-Type", "application/x-www-form-urlencoded");
            objRestRequest.AddHeader("client_id", "6hvihoiygmj1r00bs8gclv8f4jwhiqem");
            objRestRequest.AddHeader("client_secret", "<Secret>");
            objRestRequest.AddParameter("grant_type", "client_credentials");
            objRestRequest.AddHeader("box_subject_type", "user");
            objRestRequest.AddHeader("box_subject_id", "17293138289");

            RestClient client = new RestClient("https://api.box.com");
            Task<RestResponse> response = client.ExecuteAsync(objRestRequest);