Javax.Net.Ssl.SSLHandshakeException: Trust anchor for certification path not found.

Question

i have tested my SSL against https://www.digicert.com/help/ and got a green Light

"Congratulations! This certificate is correctly installed."

and my api's get this error

Javax.Net.Ssl.SSLHandshakeException: Trust anchor for certification path not found.
Java.Interop.JniEnvironment+InstanceMethods.CallVoidMethod (Java.Interop.JniObjectReference instance, Java.Interop.JniMethodInfo method, Java.Interop.JniArgumentValue args) [0x0006e] in <bd6bd528a8784b7caf03e9f25c9f0d7b>:0*

this happens in OS:Android 11 , my code is as below

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 Task.Run(() => 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;
         }

Thanks