Three different skills
Meditation is sustained, deliberate attention: on breath, a sound, a candle, or whatever keeps arriving. Visualisation is deliberately holding an imagined thing in mind. Trance describes a shifted state of attention, usually reached through repetition, rhythm, stillness or movement. Beginner books frequently use all three words for whichever one they mean.
Not everyone forms mental pictures
Aphantasia, the absence of voluntary mental imagery, is common enough that any group of practitioners will include people who have it. Instructions that say 'see a silver light' are written by people who can. If you can't, you are not failing at magical practice and you do not need to fix anything.
Other ways in
Words said or written. Bodily sensation, including warmth, weight and breath. Sound: a hum, a bell, a repeated phrase. Concepts held as meaning rather than image. Physical objects held in the hand. Movement, walking, rocking, or drawing. Any of these can carry the same work a visualisation is meant to do.
Keeping it honest
Some Wiccans understand trance as contact with something beyond themselves; others as a well-documented shift in ordinary attention. The practice can look much the same either way.
If it becomes unpleasant
Stop, open your eyes, stand up, eat something. Extended trance work is not recommended if you are unwell, exhausted, or dealing with anything that makes dissociation a concern. Practice is not treatment, and there is no prize for pushing through.