I am using this to initialize the Firebase app:
<script src="/__/firebase/init.js"></script>
To my surprise this file is missing the appId entry. When I look at the config for the project it is there (see https://support.google.com/firebase/answer/7015592).
Why is appId missing in "init.js"? How do I add it?
I strongly suspect you didn't follow all the setup instructions completely. Be sure to go back and follow them all, especially part 2: Register your app where you create an app in the console to associate your web app with its configuration.
If you've followed all the instructions exactly, please contact Firebase support directly for troubleshooting assistance.