[C#] Upload di un file

[cello182]

scusate la mia ignoranza ma nn riesco a farlo funzionare

try
                {
                    FtpWebRequest request = (FtpWebRequest)WebRequest.Create("ftp://www.*****.com/inserimento_articoli/log.txt");
                    request.Method = WebRequestMethods.Ftp.UploadFile;
                    request.Credentials = new NetworkCredential("*****", "****", "www.****.com");
                    StreamReader sourceStream = new StreamReader("log.txt");
                    byte[] fileContents = Encoding.UTF8.GetBytes(sourceStream.ReadToEnd());
                    sourceStream.Close();
                    request.ContentLength = fileContents.Length;

                    Stream requestStream = request.GetRequestStream();
                    requestStream.Write(fileContents, 0, fileContents.Length);
                    requestStream.Close();

                    FtpWebResponse response = (FtpWebResponse)request.GetResponse();

                    MessageBox.Show("Upload File Complete, status {0}", response.StatusDescription);
                    TBReport.Text = TBReport.Text + "-->Upload del file completato con successo.";

                    response.Close();
                }
                catch (System.Exception ex)
                {
                    MessageBox.Show(ex.Message, "Eccezione");
                    TBReport.Text = TBReport.Text + " -ATTENZIONE- " + ex.Message + "\r\n";
                } 

mi da il seguente errore:
Errore del server remoto: (550) File non disponibile (ad esempio file non trovato o non accessibile)..