How to setup Firebase distribution in a flutter app

Add to your /android/build.gradle file:

apply plugin: 'com.android.application'
apply plugin: 'com.google.gms.google-services'

If your /android/build.gradle file begins like this:

buildscript {
  ext.kotlin_version = '1.7.10'
  repositories {
    google()
    mavenCentral()
  }
  dependencies {
      classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
  }
}
// More code...

It is using buildscript syntax. In this case, add the plugins like this:

buildscript {
  ext.kotlin_version = '1.7.10'
  repositories {
    google()
    mavenCentral()
  }
  dependencies {
    classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
    classpath 'com.google.gms:google-services:4.4.1'
  }
}
// More code...

Assuming Firebase instruction for /android/build.gradle is to add:

plugins {
  // Add the dependency for the Google services Gradle plugin
  id 'com.google.gms.google-services' version '4.4.1' apply false
}

Source:

Firebase Documentation: Add Firebase to your android project

(The part where it says: "Are you still using the buildscript syntax? Learn how to add Firebase plugins using that syntax")