Error in launching Chrome Extension for a specific page

I'm writing a simple Chrome extension that displays a JavaScript alert saying "Hello World". In this example I have specified that the extension will only run for google.com (by putting this in the permissions property within manifest.json).

Even after everything in the target page has loaded, the alert doesn't appear. Here is my script so far:

File: manifest.json

{
  "name": "Hello",
  "version": "1.0",
  "description": "Says hello to Google",
  "permissions": ["http://*.google.com/"]
  "browser_action": {
    "popup": "Hello.html"
  }
}

File: Hello.html

<script language="Javascript">
   alert("Hello World");
</script>

You are adding a browser action popup, which adds a button to the top-right of your browser. (It's probably invisible because you haven't specified an image for it. There should be some empty space to the right of your address bar; try clicking it to see your Hello.html in a popup.)

What you want is a content script. Content scripts can get injected into every page that Chrome loads. You can use the matches and exclude_matches sub-items in your manifest file to specify which pages get your injected script.

{
  "name": "Hello",
  "version": "1.0",
  "description": "Says hello to Google",
  "permissions": ["tabs", "*://*.google.com/*"],
  "content_scripts": [
    {
      "matches": ["*://*.google.com/*"],
      "js": ["hello.js"]
    }
  ]
}

Make sure you rename Hello.html to hello.js (and get rid of the <script> tags).

Note also that I changed your http://*.google.com/ to *://*.google.com/* so that it will apply to Google over HTTP and HTTPS (and the trailing * ensures that it will apply to all pages on google.com, not just the main page).

I came across this answer trying to find a way to only enable the icon on certain pages this is how I did it. Docs

background.js

chrome.runtime.onInstalled.addListener(function() {
  chrome.tabs.onActivated.addListener(async info => {
    const tab = await chrome.tabs.get(info.tabId);
    
    const isGithub = tab.url.startsWith('https://github.com/');
    isGithub 
      ? chrome.action.enable(tab.tabId) 
      : chrome.action.disable(tab.tabId);
  });
});

make sure to add tabs permission in manifest

First of all there are 2 types of extensions:

1. Browser Action - which work for multiple websites or almost all websites
2. Page Action - which work for specific websites or webpages [which is needed 
   in our case]

Follow these steps to show your extension only on google:

Step 1: Go to manifest.json file and add the below code snippet

        "background":{
           "scripts":["background.js"],
           "persistent":false
         }
         
         ***also make sure you have page action not browser action** 
        
         "page_action" : { "default_popup":"your_popup.html" }

Step 2: Now add permissions in manifest:
        
        "permissions":["declarativeContent"]

Step 3: Now create background.js in root folder of extension and add the 
        below code in it, this will let the extension to work only on 
        urls that contain google.com
        
        // When the extension is installed or upgraded ...
        chrome.runtime.onInstalled.addListener(function() {
         // Replace all rules ...
         chrome.declarativeContent.onPageChanged.removeRules(undefined, 
      function() {
         // With a new rule ...
         chrome.declarativeContent.onPageChanged.addRules([
         {
          // That fires when a page's URL contains a 'g' ...
          conditions: [
          new chrome.declarativeContent.PageStateMatcher({
            pageUrl: { urlContains: 'google.com' },
          })
         ],
          // And shows the extension's page action.
          actions: [ new chrome.declarativeContent.ShowPageAction() ]
        }
      ]);
    });
  });

Step 4: Now reload your extension, you'll find that your extension will work 
        only for google.com

Hope this solved your query, If Yes, then Upvote the answer Thanks!