"I eat chocolate in the afternoon."
Translation:Mangio cioccolato al pomeriggio.
The partitive article in Italian is considered optional, but that's not exactly true, as in many cases omitting it or not is part of the idiom. "Mangio cioccolato" is one of the cases where its absence feels awkward. As far as I can tell duolingo's strategy is to translate it with some/any and leave it out when there's none in English; this has cost me hearts when I inadvertently applied the same idea to French.
If you want some translation fun here's an Italian article from a grammar expert: http://www.accademiadellacrusca.it/it/lingua-italiana/consulenza-linguistica/domande-risposte/cioccolata-cioccolato
Basically, in modern Italian "cioccolata" refers to the cocoa+milk beverage (e.g. hot chocolate = cioccolata calda), while "cioccolato" to the solid cocoa based paste (chocolate bar = barretta di cioccolato), but for the latter the spelling "cioccolata" is allowed as well.