A lot of people claim that they can't visualize, or they claim that their clients can't visualize. Unless you or your client has been diagnosed by a doctor with aphantasia, that is 100% not true. For context, research across multiple studies puts aphantasia at roughly 1 to 4 percent of the general population. To put that in perspective, out of 1,000 people, that is somewhere between 10 and 40 people. Rare, but not zero. And for the auditory side, anauralia is the inability to produce an internal voice or replay sounds in the mind. Research also shows a connection between anauralia and aphantasia, meaning they sometimes occur together. Both are genuinely rare. The overwhelming majority of people claiming they can't visualize or can't hear an internal voice simply have not been shown how to access those systems yet.
There are three things you can do. 1. Accept the premise and limitation that you or the client can't visualize and give up. 2. Use the NLP synesthesia pattern to lead them from one sensory modality to open up another. That is exactly why we have the module of behavioral flexibility. 3. Pay attention to their words, catch their preferred sensory modality and their accessing cues, and then go from there. Too many times people claim they can't make pictures in their head while looking up and using words like "see," "picture," "clear," and "bright," or claim they have no inner voice while the words they choose reveal active auditory processing happening below their conscious awareness.
Use the apple test to show them they can visualize, and remember it doesn't have to be crystal clear Leonardo da Vinci type visions. Even a blur is visual. Sometimes I think when people say they can't visualize it's because they can't make a perfect picture, which can be a thread to pull on to explore if they're a perfectionist.
For the internal sounds or dialogue, use the countdown test, or have them remember their favorite song and have them first hum it out loud and then have them stop humming and see if they can still hold the sound in their mind, which 99% of the time they can.