Why can't i hold down the undo/stepbackward key ?

NisrineS  wrote I'm using it on Mac at school and it seems to work perfectly fine

It may be an operating system or keyboard setting that allows this.

Just tried to hold down 'step backwards' (ctrl-z) on Illustrator and it continued stepping backwards, undoing as long as I kept holding the keys down still don't know why it doesn't work on Photoshop...

Not sure what versions you are running where, so guessing here. But the newest version of Photoshop has added in multiple steps of undo and the ctrl-z is where it lives. So each time you click it, it moves back one step in your process.