Your Sun light is perfectly pointing down. And I guess you are rendering with Eevee or Workbench since there isn't any visible Global Illumination. So you need to rotate it a bit if you want your walls to receive any lighting.
A sun light provides light of constant intensity emitted in a single direction from infinitely far away. It can be very handy for a uniform clear daylight open-space illumination. In the 3D Viewport, the sun light is represented by an encircled black dot with rays emitting from it, plus a dashed line indicating the direction of the light.
This direction can be changed by rotating the sun light, like any other object, but because the light is emitted from a location considered infinitely far away, the location of a sun light does not affect the rendered result.
Source: Light Objects — Blender Manual
If you use indeed Eevee, you might also want to check a few things:
- Enable contact shadows in your sun light settings:
- Enable Ambient Occlusion:
- Enable Screenspace Reflections:
- Add an object Light Probe > Irradiance volume, make its spheres spread across your scene, and bake Indirect Lighting: