Translation:Today's dessert is chocolate.
@Dr_Jerry: One of the simplest sentences and English speakers still manage to complicate it with seventy one different ways to say it. Your observation is instructive indeed. No wonder non-native speakers find English so difficult. (The language is not lost but we "find" it nonetheless :=). A truly complex language IMHO)
Actually Robin's grammar is fine, but the common English phrasing isn't "of today". I could say "the grammar of Robin..." and it's fine grammatically, but sounds very stilted. Note, some menus might have "the dessert/ soup/ etc. of the day", but in conversation it would be "today's dessert" or "dessert for today" - however 的 is used here to show today's "possession" of dessert (the dessert "belongs" to today) so that is probably why Duo doesn't like "for today".