https://developer.chrome.com/extensions/getstarted#user_interface

In the manifest.json code block this is highlighted:

    "page_action": {
      "default_popup": "popup.html",
    },

It contains an syntax error (trailing comma). This would be correct:

    "page_action": {
      "default_popup": "popup.html"
    },

Sorry for the wrong category "Translation Issue", but I couldn't find a better fitting one.