"- Thanks. - You are welcome."
Translation:- Дякую. - Будь ласка.
I apologize if that sounded rude. Above, I had only asked why "Прошу" was not an acceptable alternative to "Будь ласка" in this specific verbal exchange. I had posted a month ago, so the question itself may have been updated or changed by now and I had since learned that будь ласка can be used as "Please" and "You're welcome".
It's not just Duolingo, most English > Ukrainian things I've seen use that translation. From various different books, websites and apps, this is the first time I've seen "you're welcome" = "нема за що". It might be used most often, but it doesn't seem to be the standard translation (although I suspect it's more like "it's nothing"/"don't mention it").