Dependancy error when integrating android studio project with Firebase for a google sign in feature

I'm developing an app in Kotlin and keep getting the error:

Failed to resolve: firebase-auth-15.0.0`

when trying to sync gradle. I'm trying to implement a google sign in feature.

apply plugin: 'com.android.application'

apply plugin: 'kotlin-android'

apply plugin: 'kotlin-android-extensions'
apply plugin: 'com.google.gms.google-services'

android {
compileSdkVersion 28
defaultConfig {
    applicationId "nus.is3261.kotlinapp"
    minSdkVersion 21
    targetSdkVersion 28
    versionCode 1
    versionName "1.0"
    testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
}
buildTypes {
    release {
        minifyEnabled false
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
    }
}
}

dependencies {
implementation fileTree(dir: 'libs', include: ['*.jar'])
implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
implementation 'com.android.support:appcompat-v7:28.0.0'
implementation 'com.android.support.constraint:constraint-layout:1.1.3'
implementation 'com.android.support:support-v4:28.0.0'
implementation 'com.android.support:design:28.0.0'
testImplementation 'junit:junit:4.12'
androidTestImplementation 'com.android.support.test:runner:1.0.2'
androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
implementation 'com.google.firebase:firebase-auth:16.0.1:15.0.0'

}

I have tried to follow the solution on stack overflow here, but I am left with this warning after syncing gradle Warning: The app gradle file must have a dependency on com.google.firebase:firebase-core for Firebase services to work as intended.

You are getting the following error:

Failed to resolve: firebase-auth-15.0.0

Because you are using a wrong dependency in your code. To solve this, please change the following line of code:

implementation 'com.google.firebase:firebase-auth:16.0.1:15.0.0'

to

implementation 'com.google.firebase:firebase-auth:16.0.5'

Because such a version 16.0.1:15.0.0 does not exist.

Please also add the following dependency which is now mandatory:

implementation 'com.google.firebase:firebase-core:16.0.4'

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

In your top level build.gradle file please be sure to have the latest version of Google Service plugin:

classpath 'com.google.gms:google-services:4.1.0'