It is one of the accepted answers. If it was not accepted for you, I suppose it was on a strengthen exercise? EDIT: See my second comment below about the correctness of this sentence.
I have an objection here... Imo "chocolates are sweet" shouldn't be accepted as a correct answer for "οι σοκολάτες είναι γλυκά". The fact that γλυκά is not the same gender as the noun σοκολάτες clearly shows that in this sentence it is a noun, not an adjective. "Chocolates are sweet" would be "Οι σοκολάτες είναι γλυκές". If we want to omit the article, it is "Chocolates are sweets".
Yes, we have removed this one about three months ago. See my comment above ;)
We have noticed that, for some weird reson, strengthen exercises do not accept all the translations that we have added in the incubator. We believe that this will be solved after we get out of beta and, unfortunately, there is nothing we can do about it at the moment... :/ Sorry for the inconvenience...
Now I see that you wrote "chocolates are sweet". If that was your answer, then it is wrong, because γλυκά is not an adjective, its the noun "sweet", so sweet should be in plural. If we wanted to say "chocolates are sweet" ie have a sweet flavor, it would be "οι σοκολάτες είναι γλυκές (feminine plural nominative)" not γλυκά (which is neuter plural nominative). Either way, the aforementioned problem exists and you may encounter it in other exercises. Keep feedbacking, though! There may be something that we missed, of course! ;)
The Beta is over, I think, but the error still occurs to me. Do you know why?
I think the problem here is that "the chocolates are sweets" doesn't really work as an English sentence. You could make a general statement, that chocolates are a type of sweet, but that would be "Chocolates are sweets"