Why am I getting the 'FCM registration error: Failed to retrieve token' on Android?

This error message indicates that your application is unable to get a push token from Firebase Cloud Messaging (FCM). This is usually caused by an incorrect Firebase configuration in your Android project.

Here are the most common configuration issues to check:

1. Verify google-services.json

Ensure that the google-services.json file you downloaded from your Firebase project is placed in the correct directory:

• Directory: your_project/android/app/
• Content: Make sure this file corresponds to the correct Firebase project and that the package_name inside the file matches your application's package name.

2. Apply the Google Services Gradle Plugin

The Google Services plugin needs to be correctly applied in your Gradle files. Modern Android projects may use either Groovy (.gradle) or Kotlin DSL (.gradle.kts) for their build scripts. Make sure you use the syntax that matches your project setup.

For Groovy (build.gradle files):

1. In your project-level android/build.gradle file, add the google-services classpath dependency:

   buildscript {
       dependencies {
           // ... other dependencies
           classpath 'com.google.gms:google-services:4.3.15'
       }
   }
   

2. In your app-level android/app/build.gradle file, apply the plugin at the bottom of the file:

   apply plugin: 'com.android.application'
   // ... other plugins
   
   // Apply the Google services plugin
   apply plugin: 'com.google.gms.google-services'
   

For Kotlin DSL (build.gradle.kts files):

1. In your project-level android/build.gradle.kts file, add the plugin to the plugins block:

   plugins {
       // ... other plugins
       id("com.google.gms.google-services") version "4.4.1" apply false
   }
   

2. In your app-level android/app/build.gradle.kts file, apply the plugin in its plugins block:

   plugins {
       // ... other plugins
       id("com.google.gms.google-services")
   }
   

After verifying these steps, clean and rebuild your project. This should resolve the token retrieval error.