Plataforma API 26 do Android não encontrada, pacote não disponível para download

Aqui está o meu arquivo gradle:

android {
    compileSdkVersion 26
    buildToolsVersion "26.0.0"
    defaultConfig {
        applicationId "com.loredylore.lorebuilder"
        minSdkVersion 15
        targetSdkVersion 26
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        vectorDrawables.useSupportLibrary = true
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(include: ['*.jar'], dir: 'libs')
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    compile 'com.android.support:appcompat-v7:26.0.0-alpha1'
    compile 'com.android.support:design:26.0.0-alpha1'
    compile 'com.android.support.constraint:constraint-layout:1.0.2'
    compile 'com.android.support:support-v4:26.0.0-alpha1'
    compile 'com.android.support:support-vector-drawable:26.0.0-alpha1'
    compile 'com.android.support:recyclerview-v7:26.0.0-alpha1'
    compile 'com.google.firebase:firebase-auth:11.0.2'
    compile 'com.google.firebase:firebase-core:11.0.2'
    compile 'com.google.firebase:firebase-database:11.0.2'
    compile 'com.google.firebase:firebase-crash:11.0.2'
    compile 'com.google.firebase:firebase-storage:11.0.2'
    compile 'com.android.support:support-annotations:26.0.0-alpha1'
    //new
    testCompile 'junit:junit:4.12'

}

allprojects {
    repositories {
        jcenter()
        maven {
            url "https://maven.google.com"
        }
    }
}

// Add to the bottom of the file
apply plugin: 'com.google.gms.google-services'