Duolingo is correct here.
Vous pouvez le prendre en photo pour moi ? (Can you take a picture of him for me?)
Let's compare this to a similar sentence with "lui":
Vous pouvez lui dire de s'avancer? (Can you tell him to move forward?)
Let's say we are talking about a guy named Adrien.
The first sentence is equivalent to:
Vous pouvez prendre Adrien en photo pour moi?
While the second is:
Vous pouvez dire à Adrien de s'avancer?
You can notice how in French an indirect object implies a preposition of some kind (here it's "à"). In the first sentence Adrien is indeed a direct object since he is the thing that is taken into picture.