"Aphantasia" is a recently coined word that refers to the inability to visualize images in the mind. Apparently, approximately 4% of the human population has difficulty forming mental images and about 1% can't do it at all. I'm in the 1%.
Like most people with aphantasia, it wasn't so much that I discovered that I couldn't display images in my mind - it was that I discovered that other people could. It still boggles me that people actually literally see images of sheep in their heads when they "count sheep" to try to sleep at night. And if you ask the ordinary person to picture a beach, they actually see a beach in their minds. Cool.
There apparently is a visualization test that you can take to determine whether you have aphantasia. The typical exercise is to try to picture an apple. When you do this, what do you see? Some people see a detailed picture of an apple, complete with stem and shades of colour. Me, I get nothing. No can do. Trying to do it makes my brain hurt.
I've occasionally browsed in a chat group populated by people with aphantasia, some of whom are stressed out by it. For me, it's not really a big deal: not being able to visualize is like not being able to dunk a basketball. Some people can do things that I can't. Whatever. I was born this way.
I did recognize, though, that there are some workarounds that I have had to employ in order to get around the aphantasia problem. If I need to remember a number, I need to either write it down or say it to myself over and over, or I won't retain it. I have trouble reading fiction, as I can't use descriptions provided by the writer to imagine the location in which the story is taking place. And I'm grateful that my wife takes plenty of pictures of our vacation trips, as otherwise I wouldn't recall much about them. But all of these limitations are easy for me to deal with.
From what I've read, some people with aphantasia apparently are "mind-blind" in all of their senses. I am luckier than that - as if to compensate for the lack of visual imagery, I'm really good at being able to summon up songs from my memory and play them in my "mind's ear". In general, I appear to also be good at words and pattern matching. So my inner life is full of words, music, and random connections between things. I never get bored and I consider myself fortunate. So there.