I remember people trying to do it as a kid by putting their noses in the books, and it confused the hell out of me. But then when I'd explain how to do it, they acted like I was muttering moon speak.
But what I do is just make repeating shapes overlap. Like, have you ever made your fingers overlap (by crossing your eyes)? Or played with the double images of things near your face while looking at something in the background?
You just do that so that neighbouring copies of image elements overlap.
Do you have extremely unbalanced vision in your eyes? Even with corrective lenses that can keep you from being able to stitch the images together properly for your brain to interpret the 3D image.
The size of this image is making it a little weird. This one was clearly designed to be a full-sized poster, and unless you're viewing the image on a very large display, it kinda pushes the amount of convergence that your eyes have to do into the slightly-too-small range.
That means you're likely adjusting your eyes to a point that doubles the correct convergence distance, and you're getting a garbled image.
Even when you do get it to appear correctly, the too-small size will make the illusion of depth somewhat less effective than it would be if you were looking at it, in the intended scale.
EDIT: The source for this knowledge = every book about stereograms that I could ever find. Which was weirdly only a couple that actually discussed how they work, rather than just having a bunch of them printed. But I was legit OBSESSED with stereograms, back in the 90s. I read about them the way a kid who suddenly grows past 6'4" suddenly starts reading about basketball.
Somehow, my eyes just sort of do the magic eye thing naturally. Like it's never taken me any effort, I just have to move my eyes a certain way, like I was focusing on something, and they appear.
I got it eventually. I tend to put my nose right up to the screen to uncross my eyes enough, and slowly pull it away until it comes into focus, but the right one is harder because it's supposed to be closer, and therefore bigger.
Cross eye is easier for me, but these are never cross eyed.