Smart Wi-Fi Switch with No Ground
In mains electrical wiring, ground is a safety shield.  Current never flows on ground (except during a fault) and you never, never intentionally return current there. Never. 

Mains electrical also has a "Neutral", which is analogous to electronics Vss or GND.  

A smart switch needs to power itself at all times, so it can respond to requests.  It obtains power across two wires: Neutral, and always-hot aka supply aka line.  Your smart switch needs Neutral.  

An old school steam switch does not need neutral, so it typically has two wires: always-hot aka supply aka line, and switched-hot aka lamp aka load.  

If you look in the back of the junction box, you may find some white wires in a bundle. Or light blue in modern European installations, or God knows what in older world installations. You can add the smart switch's neutral wire to that bundle of white or light blue wires. Don't break up the bundle; they are not spares. 

Not every switch box has neutral.  A few smart switches in the US tout themselves as "not needing neutral" -- however some of those do need safety ground, because what they're really doing is returning current via ground instead of the neutral they'd normally use. (didn't I just say never do that?  They got a waiver from Underwriter's Laboratories to do that thing.  The NFPA has told UL to knock it off.) 

Yet other devices can power themselves by leaking current through the lamp.  This depends on either an incandescent bulb, or an LED which is designed to tolerate this.  Those leaker types do not need neutral nor ground.  If you use one with an LED not designed to tolerate this, either the LED will fail to work, catch fire, or glow at all times. (so will the incandescent, just not bright enough to see).   With this type, you can also install a resist-- wait, DO NOT hit Mouser's website and go order a resistor, anything you install in Mains wiring must be rated and approved for that use, and Leviton and a few other makers make listed capacitive/resitive based products for that purpose.  Just use those.