"Esta caja, ¿por qué no la abres?"

Thanks. It's been corrected.

I've seen quite a few of these types of sentences in the new material and my conclusion is yes. Duo has deliberately included both "esta caja" and "la" in the Spanish sentence, so they do want you to use both "this box" and "it" in the English translation.

Your answer was marked incorrect for leaving out the word "it" in this sentence. "Por qué no abres esta caja?" = Why don't you open this box?" or "Why aren't you opening this box?"

In these sentence, it's not so much that they are teaching the meaning of individual words but they are trying to help people learn different types of sentence structures in Spanish. Having both a named object "this box" AND an object pronoun "it" seems very redundant in English so I do understand the tendency to streamline the sentence. However, that is the point. Spanish often does have redundant object pronouns in the same sentence with a named object.

You need to put what the sentence says...