Most problems are detected when Inform translates the source text, but this is one of the exceptions, a "run-time problem" which emerges only during play.

You tried to move a backdrop to a specific place or item. Backdrops can, in principle, begin play in any combination of the rooms, and most backdrops never move at all - they are like a fixed painted backcloth in a stage set. If they do move, the restriction is that they can only move to single regions, and that restriction seems to have been broken here.