"Who is the person that influenced you the most?"
Translation:Seni en çok etkileyen insan kim?
Kişi was actually what I tried, not accepted (I thought it was an ACC case marking)
"Seni en çok etkileyen kişi kim?" sounds perfectly fine to me. It's actually even better translation than the original. If you ask me it should definitely be accepted, i'd report it if I were you.
kişi has been an accepted alternative since the course was launched.