Track and change button y position

Good morning fellow Unity users.

I am attempting to create a carousel of sorts with icons that the user can drag vertically and repeat. So far I am using two windows with a separate GUI in each window and the window is draggable and repeats properly vertically.

Now I want to scale the button up as it comes onto the screen and down as it leaves the screen. Currently I am not sure how to track and change the button's y position as what I have tried in the past always throws a strange error that I can't figure out how to fix.

I copied my code to pastebin.

I have attached a screenshot of the error that is thrown when the game is ran. I also attached the js file for cleaner review. windowDrag.js

Any advise on this would be great, thanks!

Regards,

Frank