There are some other things that have to be done from the Firebase side for the AppMySite build to be able to recognize that you turned on push notifications for the Firebase App.
Here’s a good walkthrough on it:
You have to make sure the APN’s Auth Key is created in your Apple Dev account and set as well as the Web Push Certificate
You have to go to Firebase Project > Project Settings > Cloud Messaging to make sure those are done first before you attempt to build.
Figured this out earlier today and wanted to make sure others were aware.