I see two issues with this sentence, one is the verb lasciare that is clearly hard to translate without context, it can be drop, leave, or let go off. It definitely doesn't translate as leave behind which is typically used in English as 'forget to take' or 'decided not to take'... The other problem I see is that candy in English is not a countable name, therefore you can only have a piece of candy or a lot of candy, but not 'a candy'. My best translation would be 'she leaves a piece of candy'.
Candy can be both singular or plural. http://www.collinsdictionary.com/dictionary/american/candy?
I'm afraid you misunderstood. When I said both singular and plural and gave a reference from a reliable dictionary I meant singular and plural. Check the dictionary. "A piece of candy" is making the uncountable noun countable which is also correct and indeed more common. Btw I'm American and an English teacher but I used the dictionary for support because it's hard to make a point on an impersonal venue only stating personal knowledge. http://www.merriam-webster.com/dictionary/candy that's the American and here's the British: http://dictionary.cambridge.org/dictionary/british/candy Ok, a bit of overkill I apologize.
I'm not sure, but I could suggest that it's because they're trying you to learn one translation of "lascia", and you're typing in a different translation. Once I tried typing in "second year university student" for a word that was supposed to mean "bean", and it didn't accept it. I think that is the same problem you're having.
The problem is the translations being done by Italian speakers rather than English speakers. Translation (like interpretation, which is spoken) should be ONLY into one's NATIVE language. Otherwise you get things like Duolingo's "You will be able to recall more accurately from your brain." If I translated that back into English, it might be correct Italian -- but it's not correct English. I teach French so I understand this problem quite well. Example: "my leg, she.." is good French but not English! In my experience it's pretty easy to tell when a French speaker translates into English -- on a website for example. Or a Chinese translates into English in directions for a product.
English speakers are indeed use "candy" as a generic reference to some form of sweet treat with a very large amount of sugar in it and almost all the time refer to individual candies as "pieces of candy" or "a piece of candy", but "a candy" is a "thing" in English, even though it's archaic and very little used in both written and spoken forms.
I see where you're coming from...in French, the verb 'laisser' is like the Italian 'laciare'. In French, it's used both for 'to let'(rather than to approve, although similar), and 'to leave'. Im assuiming it's the same for this verb in Italian. For example, 'Je te laisse aller.' = 'I let you go.' Whereas 'J'ai laissé mon portable!' = 'I left my phone!' Kind of see the difference? There's just different ways of using the verb, depending on the context in which it is used.
If the point is to show singular/plural candy/candies, in Midwestern U.S. English, one would say "a piece of candy" to specify a singular item. Generally saying "candy" can be singular or plural "Candies" would indicate more than one KIND of candy, which also could be included in the word "candy." All this to say my response should be correct.
Unfortunately, that happens sometimes on Duo (not too much) and the only way you learn it is when you lose a heart and see the correct answer. It has a lot to do with the sentences being corrected by computer programs. A very good policy is to Report the mistake. I've done so. In addition I've seen many things corrected after a while.