Again, please follow the docs. You have mixed up things. This service file you use is intended for the user instance. So place it in ~/.config/systemd/user, and start it as the current user with
$ systemctl --user start syncthing.service
When you really want to use it as root, then use the system file. Place it in the appropriate load path, /etc/systemd/system, and type:
# systemctl start syncthing@root.service
But as calmh already outlined, don’t do that…