I first became aware that there are people who cannot visualise (form internal pictures in their mind's eye) when leading visualisations (a common practice in Wicca), and I now routinely test people when they visit or join my circle to see if they can visualise. I do this by asking them to imagine an orange: looking at it, feeling the pitted surface, opening it with their thumbs, smelling the scent orange oil rising from the peel, and tasting the fruit. If you can't visualise that, chances are you have aphantasia.
If I find someone who cannot visualise, I adapt the way that I do visualisations to help them feel included, for example by inviting them to imagine the physical or emotional feelings associated with the experience.
A new article on Book Riot, What do you see when you read? discusses the phenomenon of aphantasia in relation to reading and visualising the scenes described in books, and the realisation that not everyone sees the same thing in their mind's eye.
This learning experience makes me realize we should constantly strive to understand how people around us perceive the world. It may be different from our perceptions. Don’t assume when your friends read the same book they saw what you saw. We may all model reality differently, in a myriad of ways with different kinds of cognitive functionality. Visualization is a spectrum, but so are most perceptions.
Exeter University has developed a test for aphantasia, and you can take it via the BBC website.