Why is elle met les tasses là not accepted for she put the cups there?

It's simply because the lesson is about the pronoun "y". You're supposed to practice its use. 

-- Chris. 

In general, you are doing something very tempting when learning a foreign language, but something you can never do: you can never take an English sentence, translate each word with a dictionary, and end up with a correct foreign-language sentence. That's just not how grammar works. Foreign languages not only have different lists of words--they have different ways of putting words together into sentences.

There is a famous book where someone tried to go the other way: he took French sentences and looked up the matching English words in a dictionary. The results were pretty comical:

https://en.wikipedia.org/wiki/English_As_She_Is_Spoke