In which file did you encounter the issue?

/GoogleCloudPlatform/python-docs-samples/blob/master/appengine/standard_python37/building-an-app/building-an-app-3/templates/index.html

Did you change the file? If so, how?

This solution will work:

1. Paste the initialization snippet in the part of html page before the alert.
2. Change the version of firebase ui to 4.2.0:
   <script src="https://cdn.firebase.com/libs/firebaseui/4.2.0/firebaseui.js"></script>
<link type="text/css" rel="stylesheet" href="https://cdn.firebase.com/libs/firebaseui/4.2.0/firebaseui.css">
3. add this line at the :
   <script src="https://www.gstatic.com/firebasejs/6.2.4/firebase-auth.js"></script>
   AND PROBLEM SOLVED.
   The problem is the version in the example code is too old.

Describe the issue

When I wanna to add firebase to my web app, the webpage cannot have the index page showing the icon according to the tutorial: Adding Firebase to your GCP project