This wouldn't be said in English as far as I know. At least not the most common way to say it. The office is totally/completely clean would be what I would expect to hear.
How is "completely clean" not allowed here?? There's even another sentence in this model where the English "completely clean" is translated as absolutamente limpo and Duolingo marks it right.
Maybe every time you have seen it, it is spotless, so you assume that it is always that way.
When I hover over absolutamente the hint words I get are 'absolutely' and 'not at all' can this be correct?
You can use «a secretária» or «a escrivaninha» for the large ones with drawers found in offices or in the front of classrooms.