The light placement tool does not allow you to place lights below 48 px on the Z layer, you have to make that adjustment in the light editor.
It’s slightly irritating since I like to use low walls for the bottom portions of my maps, where a light is used in place of a doorway; and I have to place the light low or else it’ll illuminate the roofs of the walls, which I don’t want.
Allow Lights to be placed on Z 16 in map editor