"What do you think of his songs?"
Translation:Wie findest du seine Lieder?
I'm not totally sure. If you talk about duolingo maybe it is because of "wie"
I'm not a teacher for german (I just speak this language^^) but I would say "Wie denkst du über seine Lieder." is more colloquial. Try "Was denkst du über seine Lieder." This should be accepted.
best regards Angel