Translation:In the afternoon, we are going to the shop to buy a gift.
Should also accept "In the afternoon we'll go shopping for presents".
I think many of these are devised by Chinese speakers with decent English but overly rigid ideas about English word order since they often seem to insist the time phrase comes last. At least until we suggest enough other acceptable answers.
Ideally the courses should be made my both native English speakers with decent Chinese and native Chinese speakers with decent English. Native bilinguals in both languages being probably too hard to find.
As is often the case Duolingo rejects a translation whose word order implicitly places a slight emphasis on a component that the arbitrary compiser thinks cannot receive emphasis. These arbitrary decisions reflect badly on the whole course, which should be polished carefully to eliminate such unjustified exclusion. I happen to have placed the time phrase in a location which would be perfectly acceptable in the Queen's English, but was somehow found objectionable to a Duolinguist, apparently without cause!
These translation exercises are plagued by the fact that there is no morphological distinction between singular and plural in Chinese. I said "gifts" in this one because I got a previous, similarly-structured one incorrect because I said "a hat" when the system wanted "hats" (which didn't make sense contextually, since who goes to the store to buy multiple hats at once?).
"We'll go to the shop" is unnatural. "The shop" is usually used to refer to a car or barber shop. It should be "We're going to the store" or "We'll go to the store." Also, gift/present should be interchangeable. The Chinese does not specify whether it's singular or plural gift/presents.