This error message appears as a result of an unsuccessful test connection. First thing is to double-check the credentials from the FTP settings tab. There's a chance that one of these values was entered with a typo, either the password or user name is wrong, port is missed, or there's an extra-spacing in one of these fields.
Make sure that the credentials correspond to the SFTP/FTP settings of the platform you're uploading the feed to:
If the credentials are confirmed to be valid, yet the problem remains, it could be an internal server issue that causes this error. Please make sure that the firewall rules or, in general, the server allow connecting to the specified host and port and open for outbound connections. Once it's enabled, the problem should be resolved.