How to bypass minimal load limitation on a power switch?
That unit uses a two-wire connection so it relies on being able to pass a little current through the load while "off". The load resistance needs to be low enough that enough current can pass without a large voltage drop across the load and without turning on the load. They have chosen 20 W.

Options are adding in a parallel load, using a > 20 W lamp or, possibly, a capacitor load. The capacitor would pass some current but since this would be out of phase with the voltage it would not get hot. Whether or not the switch would like this or not is impossible to say.