I had the same problem. I deleted the newly created Aux Bus (env_corridor), then created a new aux bus. Before assigning an effect to that aux bus, I went and routed the audio from the main character (as instructed) to that aux bus. Then, I added the reverb effect to the new aux bus I created, and renamed it env_corridor again.
Later in the lesson you'll have to undo that anyway, so it's all for naught.
Hope this helps