AuthenticationError.NOT_ADS_USER

I keep getting this error

"<?xml version=\"1.0\" encoding=\"UTF-8\" standalone=\"yes\"?><reportDownloadError><ApiError><type>AuthenticationError.NOT_ADS_USER</type><trigger>&lt;null&gt;</trigger><fieldPath></fieldPath></ApiError></reportDownloadError>"

I know this error means:

Reason

NOT_ADS_USER

Summary

The login used to generate the access token is not associated with any AdWords account.

Common causes

The login information provided corresponds to a Google account that does not have AdWords enabled.

The problem is that I am able to use these same credentials in the C# sample asp.net application and get authenticated. 

Why would these credentials work in one instance but not this instance?

Here is my code:

using RestSharp;

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

namespace GoogleAdWordsReportPuller

{

    class FileGrabber

    {

        string reportDefinition = @"<reportDefinition xmlns=""https://adwords.google.com/api/adwords/cm/v201603"">

  <selector>

    <fields>AdGroupName</fields>

    <fields>Date</fields>

    <fields>AdGroupStatus</fields>

    <fields>CampaignName</fields>

    <fields>Impressions</fields>

    <fields>Clicks</fields>

    <fields>Conversions</fields>

    <fields>Cost</fields>

  </selector>

  <reportName>Custom Adgroup Performance Report</reportName>

  <reportType>ADGROUP_PERFORMANCE_REPORT</reportType>

  <dateRangeType>YESTERDAY</dateRangeType>

  <downloadFormat>CSV</downloadFormat>

</reportDefinition>";

        string developerToken = "xxxxxxxxxxxxx";

        string clientId = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.apps.googleusercontent.com";

        string accessToken = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"; // actually a refresh token

        string accountId = "585-353-4323"; // client account id

        string clientUrl = "https://adwords.google.com/api/adwords/reportdownload/v201603";

        public void GetReport()

        {

            //Host: adwords.google.com

            //User-Agent: curl, gzip

            //Accept: */*

            //Accept-Encoding: gzip

            //Authorization: Bearer [Enter OAuth 2.0 access token here]

            //developerToken: [Enter developerToken here]

            //clientCustomerId: [Enter clientCustomerID here]

            //Content-Length: 784

            //Expect: 100-continue

            //Content-Type: multipart/form-data; boundary=------------------------12d01fae60c7b559

            var client = new RestClient(clientUrl);

            var request = new RestRequest(Method.POST);

            // set http headers

            request.AddHeader("Host", "adwords.google.com");

            request.AddHeader("User-Agent", "curl, gzip");

            request.AddHeader("Accept", "*/*");

            request.AddHeader("Accept-Encoding", "gzip");

            request.AddHeader("Authorization", accessToken); // tried  request.AddHeader("Authorization", "Bearer " + accessToken); 

            request.AddHeader("developerToken", developerToken);

            request.AddHeader("clientCustomerId", accountId);

            request.AddHeader("Content-Length", "784");

            //request.AddHeader("Expect", "100-continue");

            request.AddHeader("Content-Type", "multipart/form-data; boundary=------------------------12d01fae60c7b559");

            //set request parameters

            request.AddParameter("__rdxml", reportDefinition); // adds to POST or URL querystring based on Method

            // execute the request

            IRestResponse response = client.Execute(request);

            var content = response.Content; // raw content as string

        }

    }

}

I'm wondering if I need an access token instead because according to the documentation: https://developers.google.com/adwords/api/docs/guides/reporting#complete-example

The http header has a field that says: 

POST /api/adwords/reportdownload/v201601 HTTP/1.1  Host: adwords.google.com  User-Agent: curl, gzip  Accept: */*  Accept-Encoding: gzip  Authorization: Bearer [Enter OAuth 2.0 access token here]  developerToken: [Enter developerToken here]  clientCustomerId: [Enter clientCustomerID here]  Content-Length: 784  Expect: 100-continue  Content-Type: multipart/form-data; boundary=------------------------12d01fae60c7b559

Can anyone help me?

Thanks

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