This is something I'm struggling with too but doing some practices seems to help. For me usually when I think of picturing something, it's not so much a vivid picture that my eyes see, but something that i'm thinking of in the mind. Which I've wondered if it's pretty much the same for everyone, do you actually "see" the image when you're visualizing or is it just something you think of? Sometimes my visuals can be very vivid, as if I'm looking through my eyelids, but those are when I'm on the verge of sleeping and most of the time I hardly notice it because I'm just tired and about to go unconscious.

One exercise that I liked was using visual memory to build up visualization skills. You can do this anywhere to practice. What you do is you look at a scene for a little bit and try to see what's around you. Then close your eyes and bring back everything you can remember. I like to focus on single things at a time and get a clear idea of what it looked like, and usually I move my eyes to the same spot where I think I actually saw it so I can put the pieces together to form the full image. Remember everything you can and then open your eyes again to see how much you got right. For me at least there's usually A LOT that I overlooked, so then I close my eyes again and add those to the original mental picture I had formed, and do this over and over again until you feel like you've gotten an image that's similar to the actual scene.

Now I don't know how it is for everyone, but it seems like it helps me, and it's useful for creating images if I'm daydreaming because I have images that I've already visualized. If you say you can't picture ANYTHING then I don't know how much this will help you.

If you really can't picture anything or even have an idea of a picture at all then maybe start with very simple shapes / lines / colors and start slow to see how much you can visualize. Try to imagine a straight line, rotate and change the size, now add more lines to make a triangle, fill it in with a color, add more shapes, etc.

I hope this helps but if not then I don't know what to tell you