Unable to cast object of type 'System.Threading.Tasks.Task`1[System.Collections.IEnumerable]' to type 'System.Collections.IEnumerable'

Hi, I need to call api to retrieve some transactions details from 3rd party application into acumatica erp. For testing, I used below code in my own customization project. This is a POST request and url is also defined there. 

public virtual async Task<IEnumerable> InsertSelectedLines(PXAdapter adapter)
        {
foreach (PXResult<SOLine, SOOrder> result in POrdersView.Select())
                {

                        if (lineExt.Selected1 == true)

                        {
                            APProformaItemList toBeInserted = new APProformaItemList();

                            toBeInserted.Ponbr = order.OrderNbr;
                            toBeInserted.LineNbr = lineNbr++;

                            var url = String.Format("https://sandbox.corporate- 
                             api.hsbc.com/mock/v2/transactions");
                            HttpClient client= new HttpClient();   //added reference

                            client.DefaultRequestHeaders.Add("ContentType","
                            application/json");

                            client.DefaultRequestHeaders.Add("x-report-type", "JSON");

                            string data = "test=something";
                            //var stringContent = new StringContent(json, Encoding.UTF8, 
                             "application/json");
                            StringContent queryString = new StringContent(data);
                            HttpResponseMessage response = await client.PostAsync(url, 
                            queryString);
                            response.EnsureSuccessStatusCode();
                            string content = await response.Content.ReadAsStringAsync();
                            if (response.StatusCode == HttpStatusCode.OK) 
                            {

                                toBeInserted.Description = "test if";
                            }
                            else
                            {
                                //throw new Exception(content);
                                toBeInserted.Description = " else";
                            }



                        }
                }

            }
            return adapter.Get();

Above code defined inside a method which is used to add values from smart panel to custom screen details level. I just need to test whether this is working or not, that’s why I’m assigning some testing values to description field. But the time I adding values from the screen, I’m getting the error-

“Unable to cast object of type 'System.Threading.Tasks.Task`1[System.Collections.IEnumerable]' to type 'System.Collections.IEnumerable'”

Can someone help me to find a solution to this or provide me the guidance to get this  done?

Thank you.