I’ve gone to layouts, checked show diagram and nothing is showing up, doesn’t matter if I select diagram or note names, nothing, and it insists on playing the notes of C major through the gliss because I can’t change the diagram because the diagram won’t appear. Don’t know what to do here, all help says to do what I’ve already done
Looks like you haven’t created a harp pedal diagram there. Select the first note and go to Write->Calculate Harp Pedals. Alternatively you can create Harp Pedals by invoking the Playing Technique Popover with Shift+P and then entering for instance ^- -|^^-v into it ( or in your case -^^|^- -^) to create it manually or alternatively this way.
…or, indeed, this way: