A good number of folks are going to find that they can not wired the switch.
For example, all the lights in my house only have the hot hot wire (black) to the switch and it returns to the fixture with the white (that is hot when switch is on). Therefore, no white (neutral wire) just the ground (green). The neutral (white) stops at the fixture.
I wish that this was not the case.
Before you buy look at the wiring in switch box to see if you have the white wire that is not connected to the existing switch.
This is the case with most smart switches. Without a neutral wire, there is no way to (safely) complete a circuit without running power through the light, meaning to power the switch when off, it would need an internal battery or supercapacitor that could potentially run down if the light were switched off long enough).