could not resolve hostname

This mistake indicates that the hostname you are trying to connect to can not be fixed to an IP address. (Hostnames are dealt with to IP addresses by a DNS (Domain NameServer)). 

Please check what you have entered in the Address field. You will require the legitimate hostname of an FTP server or a legitimate IP address. If you are linking with the faves, only the name or address of the FTP server belongs in the host domain as well as you must not consist of URL or path details in this field. 

If you have actually validated that the hostname is correct, there may be an issue with your DNS server (or with your ISP's DNS server, if you are a standalone individual).

 It might be unable to settle hostnames to IP addresses. At this point, it would be a good idea to contact your ISP regarding the issue.