"Stand up because we are going to arrive."
Translation:Lève-toi parce que nous allons arriver.
Duo is picky about this. In the real world le futur proche (nous allons arriver) and le futur simple (nous arriverons) are often interchangeable. Duo insists that "going to do" be translated as aller faire. Whether that's good for us learners or merely pedantic is debatable. It does keep students from avoiding learning le futur simple, which is harder than just using aller+infinitive, but you haven't done that here.