If you were invoking the FTP via the File > Export command, you must have been using an add-on plugin, either a free one that Adobe distributes or a third-party one.
It's possible the plugin is installed but disabled. Go to File > Plug-in Manager and scroll through the list of plugins on the left to see if it is listed. If it is, select the plugin and then click Enable in the Status panel on the right side of the window.
If there is no FTP plugin installed, you'll have to download one. Many people use a free one that Adobe distributes: https://jennifermack.net/2016/05/30/easy-ftp-export-from-lightroom-6cc/