Why does the Web Api request always gives me a 401 Error?

 public async void Authenticate()
        {

            AuthenticationContext authContext = new AuthenticationContext(_authority, false);
            ClientCredential credentials = new ClientCredential(_clientId, _appKey);

            _authResult = authContext.AcquireToken("https://graph.windows.net", credentials);
            var token = _authResult.AccessToken;
            using (HttpClient httpClient = new HttpClient())
            {
                httpClient.BaseAddress = new Uri(_serviceUrl);
                httpClient.Timeout = new TimeSpan(0, 2, 0);
                httpClient.DefaultRequestHeaders.Add("OData-MaxVersion", "4.0");
                httpClient.DefaultRequestHeaders.Add("OData-Version", "4.0");
                httpClient.DefaultRequestHeaders.Accept.Add(
                    new MediaTypeWithQualityHeaderValue("application/json"));
                httpClient.DefaultRequestHeaders.Authorization =
                    new AuthenticationHeaderValue("Bearer", _authResult.AccessToken);

                HttpResponseMessage accountResponse = await httpClient.GetAsync("/api/data/v8.0/accounts?$select=name&$top=3 HTTP/1.1");
                if (accountResponse.IsSuccessStatusCode)
                {
                    JObject jAccountResponse =
                        JObject.Parse(accountResponse.Content.ReadAsStringAsync().Result);
                    _logger.Information("API TEST");
                }
                else
                    return;

            }
        }