I suggest you just use a 20-sided dice, and then…
Place down a property called “roll”, and make it 0 and numerical by default.
Button pressed (Game Overlay) → Trigger (Trigger)
The trigger goes:
Set property [roll]
Value (random integer) from [1] to [20].
And then everything else.
However, if you dont want to do that, use this guide to make a popup shop, and then when each call-to-action button to choose the dice is clicked, set a property called “sides#” to that corresponding value using counters and repeaters. Then, do the thing with the trigger again but instead…
Set property [roll]
Value (random integer) from [1] to (get property) [sides#]
And then do everything else.