Failed to resolve: com.google.firebase:firebase-core:16.0.1

From the docs:-

Your app gradle file now has to explicitly list com.google.firebase:firebase-core as a dependency for Firebase services to work as expected.

Add:

 implementation 'com.google.firebase:firebase-core:16.0.1'

and in top level gradle file use the latest version of google play services:

classpath 'com.google.gms:google-services:4.0.2'

https://firebase.google.com/support/release-notes/android

https://bintray.com/android/android-tools/com.google.gms.google-services

Note:

You need to add the google() repo in the top level gradle file, as specified in the firebase docs and also it should be before jcenter():

 buildscript {
  repositories {
          google()
          jcenter()
      }



dependencies {
  classpath 'com.android.tools.build:gradle:3.1.3'
  classpath 'com.google.gms:google-services:4.0.2'
   }
}

allprojects {
     repositories {
              google()
             jcenter()
  }
}

task clean(type: Delete) {
  delete rootProject.buildDir
 }

https://firebase.google.com/docs/android/setup

Since May 23, 2018 update, when you're using a firebase dependency, you must include the firebase-core dependency, too.

If adding it, you still having the error, try to update the gradle plugin in your gradle-wrapper.properties to 4.5 version:

distributionUrl=https\://services.gradle.org/distributions/gradle-4.5-all.zip

and resync the project.

Add maven { url "https://maven.google.com" } to your root level build.gradle file

repositories {
    maven { url "https://maven.google.com" }
    flatDir {
        dirs 'libs'
    }
}