Translation:Please wait for me for 20 minutes.
It is in English, though it does have a different meaning that way. The duration of the wait (for 20 minutes) is stressed in your sentence, but the translation Duolingo suggests stresses what to wait for (me). I do not know if Mandarin has a way to create that effect too. Anyone have an answer for that?
The tips for this lesson just say that "分 is short for 分钟".
It seems reasonable to me that when specifying a full time of day like "11:45 AM" you'd want to use the short form, and when you say "wait 20 minutes" you'd want the long form because the word "minute" is a bit more important here.
After all, in English we're even more lazy and say "eleven forty-five" rather than "eleven hours and forty-five minutes" for the full time - but it's very informal at best and also really weird to say "wait five for me".
But this is just a guess on my part.
The entire Chinese course here is plagued with this problem, and there are many pages on discussions that repeat this over and over again. But nothing ever seems to change.
It basically means you get stuff wrong all the time unless you learn the answers by rote. It is demoralising to get marked wrong when you know that the English sentence you choose is not different in meaning to the answer they give. We are not here to learn English.
I am in favour of doing the most exact translation because I think you learn more about Chinese phrasing and syntax this way, and get it into your head. In this case I think the answer should be: 请等我二十分钟 = Please wait me 20 minutes. Totally comprehendible in English although a little different, but it mirrors Chinese thinking around the sentence construction and grammar.
Think of 分钟 as the full word, and 分 as an abbreviation. Remember that in the context of money, 分 means 'cents'. Actually, 分 by itself has all sorts of uses related to dividing, distinguishing, small pieces: one tenth of certain units of measure, a point scored in certain sports, etc. So you're saying 分钟 to be clear. (钟 means 'bell' and therefore a unit of time.)
This comment says that durations come before inanimate noun objects, but after animate and pronoun objects.
I can kind of understand this for pronouns because they're short and semantically "light." Both the duration and the object have to be connected to the verb, and it's easiest on the listener if they're as close to it as possible. The total strain is less if you put the shorter one first. But that doesn't really explain why animates should behave the same, unless they're just riding the semantic coattails of the pronouns.