Firebase Authenticationの実装(Google認証)　getString(R.string.default_web_client

前提・実現したいこと(

グーグルアカウントを用いた認証をするために
Android で Google ログインを使用して認証する
 Firebase Authenticationの実装(Android, Google認証)
上記のサイトを見ながらコード入力をしていました。

しかし以下のコードでエラーが発生しました。

private fun configureGoogleSignIn() {
        mGoogleSignInOptions = GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
            .requestIdToken(getString(R.string.default_web_client_id))
            .requestEmail()
            .build()
        mGoogleSignInClient = GoogleSignIn.getClient(this, mGoogleSignInOptions)
    }

default_web_client_idがunresolved referenceと出たので、調べたところgoogle-services.jsonの配置とbuild.gradleの記述が間違っているときにエラーがでるらしいのですが、確認しても間違いが見当たりません。
もし原因がわかる方いらっしゃいましたら、教えていただきたいです。

build.gradle(app)

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'

android {
    compileSdkVersion 29
    buildToolsVersion "29.0.3"

    defaultConfig {
        applicationId "com.example.diaryapplication"
        minSdkVersion 21
        targetSdkVersion 29
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }

// To inline the bytecode built with JVM target 1.8 into
// bytecode that is being built with JVM target 1.6. (e.g. navArgs)


    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
    kotlinOptions {
        jvmTarget = "1.8"
    }

}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    implementation 'androidx.appcompat:appcompat:1.1.0'
    implementation 'androidx.core:core-ktx:1.2.0'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    implementation 'com.google.android.material:material:1.1.0'
    implementation 'androidx.navigation:navigation-ui-ktx:2.2.2'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test.ext:junit:1.1.1'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
    implementation 'androidx.gridlayout:gridlayout:1.0.0'
    implementation 'com.google.firebase:firebase-analytics:17.4.1'
    implementation 'com.google.firebase:firebase-database:19.3.0'
    //noinspection GradleCompatible
    implementation 'com.android.support:cardview-v7:28.0.0'
    //noinspection GradleCompatible
    implementation 'com.android.support:recyclerview-v7:28.0.0'
    implementation 'com.google.android.gms:play-services-auth:18.0.0'
    implementation 'com.google.firebase:firebase-core:17.4.1'
    implementation 'com.google.firebase:firebase-auth:19.3.1'
    implementation 'com.google.android.gms:play-services-auth:18.0.0'
    implementation 'com.google.firebase:firebase-firestore:21.4.3'

    //play service auth
    implementation 'com.google.android.gms:play-services-auth:18.0.0'
}
apply plugin: 'com.google.gms.google-services'

build.gradle(Project)

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
    ext.kotlin_version = '1.3.61'
    repositories {
        google()
        jcenter()
        
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.6.3'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        classpath 'com.google.gms:google-services:4.3.3'

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

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

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