I’ve big difficulty to visualise anything in my mind.
It’s almost always pitch black, or sometimes very very very fainted.
I try the Hell's Army Spiritual Warfare Training Manual (First Month: Days Ten through Twenty-Four, Visualization Training Exercises p. 11) but I cannot imagine even the simplest form.
I’m beginning to wonder if I have some medical issue which prevent me from doing what every other human seems to be capable of…
Does someone have an idea ?

Can you recall memories? Dreams? That is visualisation. Recall the face of a parent, a friend, a dog, a cat, all that is visualisation.

You can do magickal workings to Improve your visualisation skills as well.

Are you doing Void meditation?