I had a similar issue following Firebase's online guide found here.
The section heading "Initialize multiple apps" is misleading as the first example under this heading actually demonstrates how to initialize a single, default app. Here's said example:
// Initialize the default app
var defaultApp = admin.initializeApp(defaultAppConfig);
console.log(defaultApp.name); // "[DEFAULT]"
// Retrieve services via the defaultApp variable...
var defaultAuth = defaultApp.auth();
var defaultDatabase = defaultApp.database();
// ... or use the equivalent shorthand notation
defaultAuth = admin.auth();
defaultDatabase = admin.database();
If you are migrating from the previous 2.x SDK you will have to update the way you access the database as shown above, or you will get the, No Firebase App '[DEFAULT]'
error.
Google has better documentation at the following: