"To which post office are you going?" - is not accepted. Is it correct translation?
It's not proper English. The correct form of the question is which... are you going to. I think it could be accepted because it doesn't change anything with regards to the Czech sentence, but they are very strict about English questions in this course.
I am English, and would say that, "To which post office are you going?" is perfectly good English, at least in the UK. In fact, "Which post office are you going to?" is a sentence ending with a preposition, traditionally considered sub-standard English, although generally accepted in the modern language. I have reported this, as I believe zubkov's answer (and mine) should be allowed.