It is also acceptable to say "al cui portofel este acesta?" even though duolingo marks it as a wrong answer
What about "Portofelul asta este al cui?"?
It is correct and can also be used. (Some constructs are gramatically correct, but twisted or impopular, however it's not the case with your solution.)
What is wrong with saying asta portofel? Asta is in the hints.