Hi ALTWang,
You will need to add a Lightmass Importance Volume to your playable area.
Although, this would resolve the issue for the warning, I would not recommend using pre-computed lighting for very large scenes. The number of calculations and amount of ram needed to build lighting for larger scenes will likely cause lightmass to crash while building (this is not an editor crash though).
Also, from the look of your scene, it looks like this is only using dynamic lighting, because there is no indirect bounce light (Global Illumination).
Thank you!
Tim