Credential implementation provided to initializeApp() via the "credential" property failed to fetch a valid Google OAuth2

This looks like a problem with NodeJS, the version 8 is no longer supported as per the documentation. Deployment of functions to the Node.js 8 runtime was disabled in the Firebase CLI on December 15, 2020 and that is most likely why you are facing the 404 error.

To migrate to a newer supported version of the NodeJS runtime, use the documentation.

Also, I do not suspect any issue with your firebase-admin version 7.3.0.

I have the same error, but I was already on Node 12. Also, the function was running well for years and now is crashing due to this :/

The only solution that worked for me was to change auth to admin from default to explicit passing of service-account.json like is described here in docs https://firebase.google.com/docs/reference/admin/node/admin.credential#cert

For those who are deploying to Google Cloud Functions, requirements:

• Node >= 10

Initialisation:

const functions = require('firebase-functions');
const admin = require('firebase-admin');

admin.initializeApp();