One pre-requisite for hypnosis is you need to be able to imagine.
Everyone can imagine – some people can have a vivid picture formed while some the image is not so clear.
Basically, this is Ok. The fact is imagination is one important faculty of our brain and it is normal for people to exercise their imagination.
If you are not able to imagine vividly, you can improve it through practice. Start with something you are familiar with, example your laptop – see its shape, colour, texture, the sound made when it is turned on etc. Look at it for a while and close your eyes to see it in your mind’s eye. Repeat a few times. Then, proceed to practise on another object. It is very simple.