Chrome extension – "Cannot read property 'onMessage' of undefined"

Question

Creating a Chrome extension.

Part of the extension is to get the current tab URL.

The only way I've found of doing so is by putting the script in background.js and having an event listener in script.js

Except that's what I thought... I've been all over Google, StackOverflow, apparently everything points to same answer of putting the script in background.js and having an event listener in script.js but I keep getting Cannot read property 'onMessage' of undefined

The code below doesn't work because it gets the Chrome ext url, not the current tab url

current_url = window.location.href
console.log(current_url)

Below is what I currently have:

background.js

chrome.runtime.onMessage.addListener(
    function(request, sender, sendResponse) {
        console.log(sender.tab ?
                    "from a content script:" + sender.tab.url :
                    "from the extension");
        if (request.greeting === "hello")
        sendResponse({farewell: "goodbye"});
    }
    ); 

script.js

chrome.runtime.onMessage.addListener(
    function(request, sender, sendResponse) {
        console.log(sender.tab ?
                    "from a content script:" + sender.tab.url :
                    "from the extension");
        if (request.greeting === "hello")
        sendResponse({farewell: "goodbye"});
    }
    ); 

manifest.js

{
  "name": "Getting Started Example",
  "description": "Build an Extension!",
  "version": "1.0",
  "manifest_version": 3,
  "background": {
    "service_worker": "background.js"
  },
  "permissions": ["storage", "activeTab", "scripting", "bookmarks", "tabs"],
  "action": {
    "default_popup": "popup.html",
    "default_icon": {
      "16": "/images/get_started16.png",
      "32": "/images/get_started32.png",
      "48": "/images/get_started48.png",
      "128": "/images/get_started128.png"
    }
  },
  "icons": {
    "16": "/images/get_started16.png",
    "32": "/images/get_started32.png",
    "48": "/images/get_started48.png",
    "128": "/images/get_started128.png"
  },
  "options_page": "options.html"
}