Missing or insufficient permission?

Looks like a permission issue.

please go to Firestore -> rules then add the below snippets

service cloud.firestore {
  match /databases/{database}/documents {
    match /{document=**} {
      allow read, write;
    }
  }
}

Note: This completely turns off security for the database!

But don't worry we are doing requests from Backend where we are authorizing the requests first before making any requests to firebase.