The exterior wall will be transparent when directly rendered, and you cannot directly lay bricks or apply materials. You can drag the floor platform, adjust its size to be the same as the wall, change the thickness to 1, rotate it by pressing the X key, then drag it over and place it next to the exterior wall, and finally drag the bricks/materials for rendering. (If there are doors and windows, several floor platforms need to be pieced together to leave the positions for them.)