Translation:I am writing to them
Um ... it can!? That's what I wrote. Maybe they've actually fixed something around here.
No, because you(pl) does not only require the object infix "-wa-" but also the suffix "-eni". > ninawaandikieni
Two wersions are proper : 1) I am writing to you 2) I am writnig to them, because pronoun for you ( all) and they is the same - wa. The problem is in Duolingo's preferences to ONLY plural forms and it is happend - always.