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.

Regions combine a number of rooms under a single name. That means that to be moved to a region would mean being inside all of those rooms simultaneously, and only objects of the kind backdrop are allowed to be multiply-present like this. (If you already knew this, then the problem is probably that you forgot to make it a backdrop.)