"Choose a plate."
Translation:Scegli un piatto.
Maybe they're wanting the imperative form for Lei, which is 'scelga', not 'sceglie' - check this on Word Reference - http://www.wordreference.com/conj/ItVerbs.aspx?v=scegliere imperativo:
Scelgie is the indicative polite form, but here we have an instruction, which requires the imperative mood. However the polite imperative is always the 3rd person subjunctive, in this case scelga. This is because you can't give orders politely, so you have to say "One might choose ..." instead. Not kidding.
If this imperative comes up in Verbs, Present 2, I think it's because although scegli as an imperative hasn't been introduced, scegli as an indicative form has been introduced. I'm not sure whether Duolingo's system can't distinguish the two or whether the contributors merely didn't think of distinguishing them in the system. Whatever the case, one way to be sure of one's ground with unfamiliar imperative forms is to select an infinitive, look it up in Wiktionary, expand the Conjugation section, and find the row of imperative forms. Here's a link to scegliere's conjugation in Wiktionary:
Right now, a total of 3 upvotes would put this comment at the top, where people are most likely to see it. (Since we don't benefit from upvotes here, I feel comfortable making the suggestion.)