Probably because you haven't read Murakami's report yet.
Rejects 'I doubt the elephant's disappearance' but suggests as a correct answer 'I doubt the elephant disappearance' which is just bad English
I'm not a native english speaker, but shouldn't it be "I doubt ABOUT/OF the disappearance of the elephant"?
No, Duolingo has got it right this time! You can however, say, "I have my doubts ABOUT the disappearance of the elephant."
It's a clunky sentence and Lysanne86 is as correct as anyone else ...