Would "there is a dead person" also work as a translation of "ada orang meninggal"?
Died = meninggal. Dead = mati. So, there is a dead person would be ... Ada orang mati.
Halo teman! I think "meninggal" is always an active verb, not an adjective. Still, it seems that your sentence has a same meaning. Considering the Duolingo sentence would be the same as "Ada orang yang meninggal.", I would suggest the English translations as "There is a person who died. / There are people who died.", but I do not know if these English expressions are really good. Please correct me if I am wrong.
Selamat belajar! :)