Kotlin 无法解决：androidx.lifecycle：lifecycleviewmodelktx：1.1.1

I'm trying to use the new Navigation Architecture Component for android and I'm getting the error Failed to resolve: androidx.lifecycle:lifecycle-viewmodel-ktx:1.1.1 when I'm defining the lifecycle_version to "1.1.1"

我基本上只是复制和粘贴文档中的内容，所以我对这里的错误已经没有概念了:(

apply plugin: 'com.android.application'

apply plugin: 'kotlin-android'

apply plugin: 'kotlin-android-extensions'

apply plugin: 'kotlin-kapt'

android {
    compileSdkVersion 26
    defaultConfig {
        applicationId "com.mobile.codgin.newnavigation"
        minSdkVersion 21
        targetSdkVersion 26
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
      }
    }

    dependencies {
    def lifecycle_version = "1.1.1"

    // ViewModel and LiveData
    implementation "android.arch.lifecycle:extensions:$lifecycle_version"
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation"org.jetbrains.kotlin:kotlin-stdlib-jre7:$kotlin_version"
    implementation 'com.android.support:appcompat-v7:26.1.0'
    implementation 'com.android.support.constraint:constraint-layout:1.1.0'
    implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:$lifecycle_version"
    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'
}