This is a mistake in our text, which is fixed. It is intended to read the "next generation Firebase Realtime database."
Currently, we only support Cloud Datastore or Cloud Firestore for each project (which we hope to fix in the future).
You see this message because you have already signed up for the Cloud Firestore beta for this project through the Firebase Console. Not when you signed up for the beta, the message appears: "Enabling the beta version of Cloud Firestore will not allow you to use Cloud Datastore with this project, especially from the corresponding App Engine application."
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