Uncaught typeerror: cannot read property 'queryselectorall' of undefined. Javascript stop working as extension.

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Javascript

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Hello, I have a problem with my javascript.
When I run it from the developer console the code is working. When I use my code to create the extension - I receive error.



Thanks in advance!

What I have tried:

var fullOffer = document.querySelectorAll('.js-offer-proceed')[0];
var len = fullOffer.querySelectorAll('.cartlist__item');

Later I'm using fullOffer variable to extract specific data from the screen without any problem.

When I add this code as extension I receive this error:
Screenshot of the error[^]

What can be the reason to run without any problems the code in developer console, but when I store the code in .js file and import it as extension to stop working?

This is the code from the manifest file:

{
  "manifest_version": 2,

  "name": "Getting started example",
  "description": "This extension shows a Google Image search result for the current page",
  "version": "1.0",
    "permissions": [
    "storage"
  ],
  
  
  "background": {
    "scripts": ["background.js"],
    "persistent": false
  },

  "permissions": [
    "activeTab",
    "https://ajax.googleapis.com/"
  ]
}

I try just to create a simple alert message when I click button and the extension is working, so I expect the problem maybe it's not in the Manifest file, but not sure?
It's actually my first extension.