Are you sure you haven’t completed it yet?
Just keep taking pictures, exiting the camera and then taking more pictures.
I’ve found that the Camera challenges function in a weird way, where once you get the proper picture, the challenge unlocks 10 or 15 seconds later. There’s a strange delay.
I had a problem with the Nothing as it Seems challenge and kept trying to get it to pop, but I guess while I was frantically taking pictures, I hadn’t seen the challenge pop and complete.
(try from the inside of the secret passage?) I think I got thst challenge to pop while behind the pile of stuff behind Alexa’s chair, but it may have been through the mirror in the secret passage, too.)