Surfacegui not showing the same for different users?

I made a Self-Checkout that scans items and then lets you pay. In testing I had no issues with it but when there were multiple people in 1 server, it completely started breaking.

For example if someone clicked "Pay" it would still show the main screen for others.

and when the payment completes, it does nothing and just is bugged like that. (It doesn't detect that the payment is completed)

I have used serverscripts everywhere, and the surfacegui is adorneed to the part that needs to show the gui, otherwhise the textbuttons wouldn't click.

What can I do to fix this?