Admob + Firebase: google-services.json is missing

After adding Firebase Admob into my Android app project, Gradle build failed with this error:

Error:Execution failed for task ':app:processDebugGoogleServices'.
> File google-services.json is missing. The Google Services Plugin cannot function without it.
   Searched Location:
  ...\app\src\debug\google-services.json
  ...\app\google-services.json

Where can I get the google-services.json?

I encountered this problem because I hadn't set up my project in Firebase yet.

I went to https://console.firebase.google.com/ and signed in. Followed the Create New Project step by step to set up the project in Firebase. After the project was successfully set up, it prompted me to download the google-services.json file. Then I copied the file to my project's app/ directory.

Then I re-synchronized Gradle and this time, no errors.