Upload a Document using Adobe Sign API - getting response with "INVALID_API_ACCESS_POINT"

Hi Everybody ,

I am trying to upoad a document for sing process using  Adobe Sign API,

then I am getting an error response from API server as 

"{\"code\":\"INVALID_API_ACCESS_POINT\",\"message\":\"Request must be made to correct API access point (e.g. use GET /baseUris).\"}"

I have created an integration key with full permission and use it in the api call.

I am following the below documentation link 

https://www.adobe.io/apis/documentcloud/sign/docs.html#!adobedocs/adobe-sign/master/api_usage/send_signing.md
Anybody have any idea about the error response, Please help me to resolve the issue.
Please see my C# code as below

---------------------------------------

HttpResponseMessage result;
string api_access_point = "https://api.na1.adobesign.com/api/rest/v6/transientDocuments";
HttpClient httpClient = new System.Net.Http.HttpClient();
httpClient.DefaultRequestHeaders.TryAddWithoutValidation("Authorization", "Bearer " + this.IntegrationKey);
httpClient.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", "multipart/form-data");
httpClient.DefaultRequestHeaders.TryAddWithoutValidation("Content-Disposition", "filename="+ Path.GetFileName(this.documentFile));

this.fileContent = new ByteArrayContent(File.ReadAllBytes(this.documentFile));
using (MultipartFormDataContent mFormData = new MultipartFormDataContent())
{
mFormData.Add(this.fileContent, "File", Path.GetFileName(this.documentFile));
result = httpClient.PostAsync(api_access_point, mFormData).Result;
if (result.IsSuccessStatusCode)
{
// To Do
string response = result.Content.ReadAsStringAsync().Result;
this.transientDocumentId = JsonConvert.DeserializeObject<TransientDocumentId>(response);
}
else
{
string response = result.Content.ReadAsStringAsync().Result;
HandleError(result.StatusCode.ToString(), response);
}
}

Thank you.