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After enabling, you will redirect to database page. In that page, select tab Indexes and add 2 Composite indexes to Database by click on Create index manually. 

Setting like 2 below screenshotThen, configure fields as 2 screenshots below:
 

Image Added

Indexes enable:

Go to Develop > Authentication, enable Sign-in method for Email/Password provider in your project to allow users access to the database, as the following screenshot:

Then, go to Database > Rules tab and set up add the following database rules like::

service cloud.firestore {
  match /databases/{database}/documents {
    match /{document=**} {
      allow read, write: 
      if request.auth.uid != null && request.auth.token.email.matches('^user_([0-9]+)@.*');
    }
  }
}

Rules table will be as the below image:

Now, you can start to use IM App with Firebase.

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