I put "Roosa, is that an opera house?"
It should be accepted grammatically. However, the authors of the course probably had the one opera house in Finland in their mind.
I have reported it. Unless and until they give us a way to distinguish definite and indefinite articles, they oughtn't be marking "an opera house" as wrong.
@samulili: I agree, but in that case they should have capitalised the word. (Sept 2020)