Could this sentence be "A clock is on the black building," or would that be "A fekete épületen egy óra van"?
I'm not a native, but I believe your translation is correct. If it wasn't accepted, report it with the "Report a problem" button. Let the course developers decide if your version should be included.
If you mean to say it like "a clock is on the black building", emphasizing the particular building, it would be "a fekete épületen van egy óra" (or maybe "egy óra a fekete épületen van", though that doesn't sound good to me).
Finally, "a fekete épületen egy óra van" would emphasize the clock: "there is a clock on the black building, not something else".
Not all buildings are houses. A hospital, for example, is a building (an épület) but not what you'd usually call a "house".