I can't answer for the Danish version, but in English the term "general soldier" does not exist. It is hard to imagine what Duo means with this statement. If they mean an ordinary soldier of the lowest rank, then it could be "private, sapper, gunner, rifleman, fusilier etc." depending on which corps or regiment the soldier belongs to; but never "general". A "general" is a high ranking army officer, but is never referred to as a "general soldier".
Why is it "generel" and not "generelle" to describe the soldier? "Generelle" was used to describe a director in another Duolingo question earlier, so when I typed "generel direktor" it marked me wrong for not using "generelle". Now here it marks me wrong for using "generelle" to describe the soldier.
Does "generel" in Danish also mean "common, ordinary, normal"; "Not specialized or limited in range of subject, application, activity, etc." or just meaning like a rank? (not specifically asking about this sentence, because I've understood that as the English is off, the Danish might also be, but I want to know when I see the word in other contexts, what I'm supposed to get from it) Or can it be used for things like "in general"?