In FileZilla Client, if message failed to retrieve directory listing is displayed, the first thing you will want to do is to identify if this issue is only occurring when attempting to connect to the FTP Server from a remote computer over the Internet, or if the error also occurs when attempting to connect to the FTP Server in your LAN. If the error only occurs when attempting to connect to your FTP Server from a remote computer over the Internet, first ensure you are Port Forwarding on port 21 to the IP address of the computer that is running FTP Server. The next thing to ensure is that your Firewalls are configured to allow traffic on Port 21.