I’ve completed the Space Shooter tutorial, so my C# code is identical to that of the tutorial (except I’ve replaced the GUIText-related code for new UI code).
My only clue as to what might be going on: In the very beginning of the tutorial, I checked/set the Build Settings, and noticed a scene mysteriously already there. I think it was titled “xx_scenes/Main.unity”. It was grayed out, with its checkbox un-clickable. Since the checkbox was un-clicked, I ignored it when I eventually built the game for the web. After getting this error the first time, I deleted said scene from that list, but the error remains after rebuilding.