Are you trying to connect to an FTP server, likely using filezilla and coming across:
Response: 500 Invalid PORT Command.
Error: Failed to retrieve directory listing
*SOLUTION*
1. Check to make sure Active or Passive mode are enabled depending on which your server uses. If you're on Windows server you likely use Active mode.
2. Check your firewall / router to ensure port 21 is forwarded to your computer or is open in general to receive connection.