Translation:De uil eet je avondeten!
Why is it "je" instead of "van jouw"?
Je is the unmarked form of jouw or "your." Van jouw = yours. Here's some helpful info on Dutch possessive pronouns: http://www.dutchgrammar.com/en/?n=Pronouns.Po01.
Can't I say "de uil eet jouw diner"?
You can say that and duolingo counts both as correct.
The form "jouw" would not have been right? Instead of "je"