[android] How to add google-services.json in Android?

This error indicates your package_name in your google-services.json might be wrong. I personally had this issue when I used

buildTypes {
    ...

    debug {
        applicationIdSuffix '.debug'
    }
}

in my build.gradle. So, when I wanted to debug, the name of the application was ("all of a sudden") app.something.debug instead of app.something. I was able to run the debug when I changed the said package_name...