Translation:Today's dessert is chocolate.
This is one of the simplest sentences in the English language and Duo still managed to find only one possible way of saying it.
"for" would be better, but an English speaker would still understand it. It's just not totally correct grammar.
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".