Lights can be palced only in floorplan (2D) and not in perspective or elevation (3D).

Turn the floorplan in 2D.

From right toolbar select the icon.

The pointer turn to light. Left click to the place to put the light (as you see below).

From new menu select "Point light".

Place another light. Press left click as you see below.

Press the light button from left toolbar or ESC to deactivate the placement of light.