US>Build Error in ? After Plug-in Update

kotlin:sample.kt
package com.se.testuser.qitacliant

US>class sample {
    // method that might return null
    funreturnNullableStr(arg:String): String?{
        if(arg=="null") return null
        return "not null"
    }

    // method that does not return null
    funreturnNotNullStr(arg:String): String {
        return "not null"
    }
}

Error: Gradle: A problem occurred configuring root project 'QiitaCliant'.
>Could not resolve all files for configuration ':classpath'.
   >Could not find com.android.tools.build:gradle: 3.0.0.
     Searched in the following locations:
         https://jcenter.bintray.com/com/android/tools/build/gradle/3.0.0/gradle-3.0.0.pom
         https://jcenter.bintray.com/com/android/tools/build/gradle/3.0.0/gradle-3.0.0.jar
     Required by:
         project:

Information: Gradle Task [clean,:app:assemblyDebug]
C:\Users\testuser\AndroidStudioProjects\QiitaCliant\app\src\main\kotlin\com\se\testuser\qiitacliant\sample.kt
Warning: (15,26) Parameter 'arg' is never used
Information: BUILD SUCCESSFUL in 41s
Informational: 0 errors
information:one warning
information:See full console output

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

buildscript {
    ext.kotlin_version = '1.1.51'
    repositories {
        jcenter()
    }
    dependencies {
        classpath'com.android.tools.build:gradle:3.0.0'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"

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

allprojects {
    repositories {
        jcenter()
    }
}

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

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

android{android{
    compileSdkVersion25
    buildToolsVersion "26.0.2"
    // Add Configuration From Here
    sourceSets{

        // This is for testing purposes.
        main.java.srcDirs+='src/androidTest/kotlin'
        androidTest.java.srcDirs+='src/main/kotlin'
    }
    // Add Configuration Up to here

    defaultConfig {
        applicationId "com.se.testuser.qitacliant"
        minSdkVersion21
        targetSdkVersion25
        versionCode1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            miniEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {

    compilefileTree (dir: 'libs', include: ['*.jar'])
    androidTestCompile('com.android.support.test.espreso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    compile'com.android.support:appcompat-v7:25.3.1'
    compile'com.android.support.constraint:constraint-layout:1.0.2'
    testCompile 'junit:junit:4.12'
    compile "org.jetbrains.kotlin:kotlin-stdlib-jre7:$kotlin_version"
}
repositories {
    mavenCentral()
}