Spanish can omit the complement after debajo when it’s obvious from context. Here, debajo is understood as debajo del enchufe (under the socket).
If you want to be explicit: pongo la silla debajo del enchufe.
Enchufar means to plug in (connect a device to electricity by putting the plug into the socket).
So enchufar el portátil = to plug in the laptop (so it can charge/run on mains power).