I answered: He cannot find the lamp in the box. Duolingo says I'm wrong. Their answer is: He doesn't find the lamp in the box. Their reasoning explains that the Esperanto original is "li ne trovas," not "li ne povas trovi." In that case, the phrase should say "li ne faras trovi". Ĉu ne? Sometimes Duolingo opts for loose translation, then sometimes they use literal translation. Kind of inconsistent. Perhaps I'm missing some nuance of Esperanto?
"li ne trovas" is perfectly valid Esperanto. Esperanto is not an English-centric conlang, so expecting a 1-to-1 word translation is as reasonable as expecting it from any other language. It is not a fault of Esperanto that English expresses the negated present tense of a verb as "does not x"; that is just a peculiarity of English. My favorite phrase expressing the qualities of English: "I before E except when not." (Your comment is probably really outdated. I wanted to respond for the sake of komencantoj who may later see it.)
Along the same line, this question seems to require a 1-to-1 translation (albeit with the word order arranged differently). "He didn't find the lamp in the box." seems like a much more colloquial expression of the idea here, or if there is a need to express the ongoing attempt to find the lamp, then "He can't find the lamp in the box." seems more natural to me.
I understand that the differences in tense may cause confusion if they accepted the colloquial English past tense. My point is that, if the need is really to translate the ideas, and not just literal word-for-word translations, this isn't a very good example to use for native English speakers, as it seems unnatural.
Or perhaps that's just my dialect of English, and others would disagree with me.