suggested that one try to imagine a very simple image, such as a cloudless sky, the sea (no ships or other coastline) and a beach. So, two lines, the shore and the horizon. I tried this without success.

Have you ever been to the beach? If so, do you remember what it looked like? If so, you're done at that point, whether you actually "see" the beach or not.

Imagination is really just a form of memory, and vice versa; some people have difficulty with it simply because they try to create an image from scratch in their mind, having no idea how to go about it and nothing to start from.

In general, when any self-help book tells you to imagine or visualize something, you're better off asking yourself if you can remember something like that, or asking yourself what something like that would look like. You don't need to consciously attempt to manipulate imagery - you just ask yourself questions that presuppose you can see something, whether you feel you can "actually" see them or not.

The underlying assumption here is that your brain is absolutely capable of manipulating visual information -- otherwise, there are a wide variety of things you simply wouldn't be able to do. However, that doesn't necessarily mean that you can consciously perceive that information, without practice at observing it. In other words, your brain can visualize, but you may not be able to see that visualization without practice.

Another common block to visualization is a conceptual one: the objection that you're not "really" seeing things because they're "not real". (e.g. someone who gets told as a kid that the things they imagine aren't real and to stop it).

Anyway, not saying that you necessarily can visualize consciously or that any of these issues is yours; just pointing out that there are a lot of reasons why a person can be able to visualize in principle while not being able to perform it in practice.

Practice is actually important, too. As a computer programmer, I have considerable practice doing black-and-white visualization of boxes and lines representing data structures, but less practice at vivid color images or anything panoramic. However, if I look at something and close my eyes, I can retain the full image for a short while, because that's something I used to practice as a kid, trying to develop a "photographic memory".