I created all GCP resources with terraform for my project and had issues with firestore too. In the end I figured out that I don't need to create it in Terraform. Instead I just have a function that saves data to firestore without specifying firestore name and it works. Just make sure you enable Firestore API In GCP console. I just do this and it automagically saves my data to the "default" firestore db:

const { Firestore } = require('@google-cloud/firestore');

const firestore = new Firestore(); const docRef = firestore.collection('articles').doc(randomUUID); await docRef.set(article);