Updating purchased script

Hi everyone,
I have purchased a script from here which is:
Purchased Script
This is my website which is working fine:
I’m adding functionality to the cart. I want when someone adds items in cart, then those items will remain there until:

  1. Checkout will happen
  2. 7 Days
    Now, when I’m doing changes in the script then the whole website is becoming blank and also no error is showing. This is the screenshot of the problem that I’m getting:
    Here you can view the problem in video form as well:
    Video LInk
    Thank you.