I just want to share a solution with you.
I got an error when I tried to bake light like this:.
Firstly, I clicked “Directional Light” from the Hierarchy, and then I changed the “Mode” from “Realtime” to “Baked” on the Inspector.
Secondly, I went to back to “Window -> Ligthing -> Scene”, and uncheck the “Realtime Global Illumination”
Finally, I dragged & dropped to “Directional Light” from Inspector to “Lighting -> Environment -> Sun Source”
,and I clicked “Generate Lighting”.That’s it 
