Yes, even though this is an American English teaching program we have made every effort to include British English. This was a slip-up. We appreciate your bringing it to our attention. Do send us anything else you notice.
When one enters 'wardrobe' Duo suggests an alternative translation 'closet', but not the other way round. This course is for learning Greek, not US English, so why the bias?
We make every effort to accept every correct form of the languages we teach. And yes while Duolingo is US English based we include closet/wardrobe/cupboard for this translation.
Duo rejected your sentence for another reason and showed you the primary translation which happened to use "closet". But the error was not the use of "wardrobe" there must have been another error.
If you check the comment just above yours you'll see that "wardrobe" was accepted a month ago.
This could be resolved if you could post a screenshot showing the exercise, your reply, and the response from Duo.