Misused header name. Make sure request headers are used with HttpRequestMessage, response

Question

Good Day all

i have the following code

// HttpClient is intended to be instantiated once per application, rather than per-use. See Remarks.
static readonly HttpClient httpClient = new HttpClient();

public async Task<List<USERS>> Get_All_Customers( )
            {

                List<USERS> results = null;
                try
                {
                    USERS m_model = new USERS
                    {
                        USER_ID = 1,
                        CURRENT_DATETIME = '2021/07/01'
                    };
                    var httpClient = new HttpClient();

                    httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));


                    var Json = JsonConvert.SerializeObject(m_model, new JsonSerializerSettings
                    {
                        ContractResolver = new DefaultContractResolver
                        {
                            IgnoreSerializableAttribute = false
                        }
                    });


                    HttpContent httpContent = new StringContent(Json, Encoding.UTF8, "application/json");
                    httpClient.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", "application/json; charset=utf-8");

 var response =  await httpClient.PostAsync(string.Format("https://www.myserver.com/api/MainCustomers_OnlineDates/Get_All_Customers"), httpContent);

                    if (response.StatusCode == System.Net.HttpStatusCode.OK)
                    {
                        var content = await response.Content.ReadAsStringAsync();
                        results = JsonConvert.DeserializeObject<List<USERS>>(content);
                    }
                }
                catch (Exception ex)
                {
                    await Task.Run(() => Crashes.TrackError(ex));
                }

                return results;
            }

and this code reports the following error in AppCentre

System.InvalidOperationException: Misused header name. Make sure request headers are used with HttpRequestMessage, response headers with HttpResponseMessage, and content headers with HttpContent objects.

OS :Android 7.1.2

Thanks