There should be something in syncthings log (if that script is configured to save a log…)
Most likely that user does not have a home folder or no permission to write in it and thus it can’t create it. You can create ~/.config/syncthing manually (beloning to the user that should run syncthing) or use -home="/some/path" to use another folder for the config.
Being root, can’t you “su - -s /bin/bash ftp”?
Whoami should reply “ftp”. Check your $HOME is where it should be, then you can run syncthing for the 1st time, stop it and finally edit the config.xml file.