URI prefix is not recognized - connecting to unix server from c# app on windows

Wanting to use my ftp client that I built in c# to grab files from a unix ftp server, my current ftp client works well with windows type of an ftp server, but trying to connect to a Unix server there is an issue being detected by my ftp client suggesting "The URI prefix is not recognized.", here is my ftp client:
//The uri is/uriString: servername.domain.com // I tried placing an ftp. at the begining but no help.
//port is: 21
 

private static string[] GetFilesToftp(string WildCard, string uriString, string etServer, string etUser, string etPassword)
        {
         string ReturnStr = "";
         string ftpServer = etServer; 
         string ftpUser = etUser; 
         string ftpPassword = etPassword; 
         
         FtpWebRequest request = null;
         WebResponse response = null;
         Stream responseStream = null;
 
         try
         {
             request = WebRequest.Create(@uriString + WildCard) as FtpWebRequest;
             //Specify we are Listing a directory
             request.Method = WebRequestMethods.Ftp.ListDirectory;
             request.Credentials = new NetworkCredential(ftpUser, ftpPassword);
 
             //Get a reponse
             //response = request.GetResponse();
             response = (FtpWebResponse)request.GetResponse();
 
             responseStream = response.GetResponseStream();
 
             //Convert the response to a string
             int ch;
             while ((ch = responseStream.ReadByte()) != -1)
                 ReturnStr = ReturnStr + Convert.ToChar(ch);
 
             //split the string by new line
             string[] sep = { "\r\n" };
             string[] Files = ReturnStr.Split(sep, StringSplitOptions.RemoveEmptyEntries);
 
             return Files;
         }
         catch (WebException wex)
         {
 
             string Message = (wex.Message != null) ? wex.Message.ToString() : "";
             if (wex.Message.StartsWith("The remote server returned an error: (550) File unavailable"))
             {
               return null;
             }
             else if (wex.Message.StartsWith("The remote server returned an error: (530) Not logged in"))
             {
               return null;
             }
             else
             {
                 throw wex;
             }
            
         }
         finally
         {
             if (responseStream != null)
             {
                 responseStream.Flush();
                 responseStream.Close();
                 responseStream.Dispose();
             }
             if (response != null)
             {
                 response.Close();
             }
         }
       
        }