It a regional issue in English. In UK English, you do hear "candies". In the US they use "candy" as a common plural (like "sheep" or "fish"). I take my hat off to Duolingo in navigating the minefield that is English. But I avoid translating μπισκότα as "biscuits", which would be the UK usage.
In this translation both "they are eating sweets" and "they are eating the sweets" are acceptable. A few sentences earlier, "the man has the the plates" is acceptable, but "the man has plates" is not. Shouldn't both translations be generally acceptable; the context being the determining factor in actual use?
I agree that "they eat the candy" shoud be acceptable here as candy is also used in a plural sense. Normally we say "a piece of candy" when we mean one and only use "candies" to refer to a collection of different types of candy. From now on I am sticking with "sweets" for τα γλύκα as it is more simple