Shouldn't indah be an option too?
Cantik is beautiful/pretty, only used to human being.
Indah is beautiful/pretty, used to describe 'non-living beings' like house, stones, clothes, etc.
So how can a flower be cantik then?
Duolingo is wrong.
The right translation should be "Bunga itu indah, tapi kamu lebih cantik."
Maybe 'cantik' can be used to plants also, but as a native speaker I never use 'cantik' for describing things. Instead I use 'indah' or 'bagus.'