"Pretty" is less than "beautiful". Similarly, in Polish "ładna" is less than "piękna" :)
But of course in general you can say "Masz ładną twarz" just like you can say "You have a pretty face", it's just not the same so I think it's OK if Duolingo doesn't accept both words.
pretty, lovely, beautiful-----all describe the SAME way yet whatever one I choose is WRONG??