500 INTERNAL_SERVICE_ERROR while Cancel Subscription (.Net Core)

[NagarajuRaghava]

Implementing PayPal Subscriptions:

1. Subscriptions via PayPal Payment standard.
2. Cancel Subscriptions via PayPal API (Without SDK).

I am facing issues while cancelling the Subscription/Recurring Payment via PayPal API. I was able to get the AccessToken via the API. Below is my code for cancelling subscriptions:

string url = "https://api.sandbox.paypal.com/v1/payments/billing-agreements/" + mySubscriberId + "/cancel";
                HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(url);

                webRequest.Method = "POST";
                webRequest.ContentType = "application/x-www-form-urlencoded";
                webRequest.Accept = "application/json";
                webRequest.Headers.Add("Authorization: Bearer " + myPaypalAccessToken);
                var request = ("note=" + myCancelNote);
                byte[] req_bytes = Encoding.ASCII.GetBytes(request);
                webRequest.ContentLength = req_bytes.Length;

                Stream strm = webRequest.GetRequestStream();
                strm.Write(req_bytes, 0, req_bytes.Length);
                strm.Close();

                HttpWebResponse resp = (HttpWebResponse)webRequest.GetResponse();
                using (Stream respStr = resp.GetResponseStream())
                {
                    using (StreamReader reader = new StreamReader(respStr, Encoding.UTF8))
                    {
                        string jsonString = await reader.ReadToEndAsync();
                        reader.Close();
                    }
                }

I get the following response everytime:

{
    "name": "INTERNAL_SERVICE_ERROR",
    "message": "An internal service error occurred.",
    "information_link": "https://developer.paypal.com/webapps/developer/docs/api/#INTERNAL_SERVICE_ERROR",
    "debug_id": "c121fd139b91b"
}

Can someone assure me if i am on the right track & what might be the cause for the issue. Thanks!