Host key wasn't verified

try {
  string sFile = ConfigurationManager.AppSettings["sFile"];
  string sFolder = ConfigurationManager.AppSettings["sFolder"];
  string sftpHostName = ConfigurationManager.AppSettings["sftpHostName"];
  string sPort = ConfigurationManager.AppSettings["sPort"];
  string sUserName = ConfigurationManager.AppSettings["sUserName"];
  string sPassword = ConfigurationManager.AppSettings["sPassword"];
  string HostKeyFingerprint = ConfigurationManager.AppSettings["HostKeyFingerprint"];
  string sessionLogPath = ConfigurationManager.AppSettings["sessionLogPath"];
  string sshPrivateKeyPath = ConfigurationManager.AppSettings["sshPrivateKeyPath"];
 
  SessionOptions sessionOptions = new SessionOptions();
  sessionOptions.Protocol = Protocol.Sftp;
  sessionOptions.FtpMode = FtpMode.Passive;
  sessionOptions.HostName = sftpHostName;
  sessionOptions.UserName = sUserName;
  //  sessionOptions.Password = sPassword;
  sessionOptions.SshHostKeyFingerprint = HostKeyFingerprint;
  sessionOptions.SshPrivateKeyPath = sshPrivateKeyPath;
  sessionOptions.PortNumber = int.Parse(sPort);
 
  using (Session session = new Session())
  {
      session.SessionLogPath = sessionLogPath + "\\log.txt";
      Console.WriteLine("Starting Session...");
 
      session.Open(sessionOptions);
 
      Console.WriteLine("Session Opened");
 
      // Upload files
      TransferOptions transferOptions = new TransferOptions();
      transferOptions.TransferMode = TransferMode.Binary;
      Console.WriteLine("Putting Files in remote folder started");
      TransferOperationResult transferResult = session.PutFiles(sFile, sFolder, false, transferOptions);
      transferResult.Check();
 
      Console.WriteLine("Putting Files in remote folder ended");
      // Print results
      foreach (TransferEventArgs transfer in transferResult.Transfers)
      {
          Console.WriteLine(string.Format("Upload of {0} in secure ftp to {1} folder:{2} succeeded", transfer.FileName, sftpHostName, sFolder));
      }
      Console.ReadKey();
  }
}
catch (Exception e)
{
   Console.WriteLine("Exception occurred: " + e.Message);
   Console.WriteLine("Inner Exception: " + e.InnerException?.Message);
   Console.ReadKey();
}