"Please pay the money today."
In Japanese the high context culture allows for omitting personal pronouns more often than not. If anything, it would sound awkward if you added あなた to some sentences.
This is a prime example. It doesn't take the answer without it so I have reported it.
That was what I wanted to know. Is Duo wrong in asking for the "wa" or is it needed for a particular reason?
No, that's wrong. The particle は does not indicate the grammatical subject. If you want to get anywhere in Japanese, you'll want to unlearn that as quickly as possible.
Geh! I've been making an effort to type solely in kana just to avoid getting docked for typing (correct) kanji, but this one was marked wrong because I DIDN'T add kanji. There's seriously no winning when you try to type the answers.
What it says, to my best understanding, is "today it's you that pay".
Ah Na Ta is used properly in THIS case when you think of it like this: Imagine someone else talking to YOU that you owe them, in this case, the YOU or AhNaTa makes sense from the third perspective.
Also, wish me good luck as I complete the entire Japanese unit!