Android studio 为什么在android studio中未指定CompileSDK版本时会出现错误？

请看左下角。错误一次又一次地显示出来。

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请帮我解决这个错误。请

您使用的是哪个版本的Android Studio和Gradle版本？

apply plugin: 'com.android.application'

android {
  compileSdkVersion 26

  defaultConfig {
    applicationId "com.debu.mrsarkar.bangalighyam"
    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 {
implementation fileTree(dir: 'libs', include: ['*.jar'])
compile 'com.android.support:appcompat-v7:26.1.0'
compile 'com.android.support:design:26.1.0'
implementation 'com.android.support.constraint:constraint-layout:1.0.2'
implementation 'com.google.firebase:firebase-database:11.8.0'
implementation 'com.google.firebase:firebase-core:11.8.0'
implementation 'com.google.firebase:firebase-storage:11.8.0'
implementation 'com.squareup.picasso:picasso:2.5.2'
implementation  'com.squareup.okhttp:okhttp:2.7.2'
testImplementation 'junit:junit:4.12'
implementation 'com.firebaseui:firebase-ui-database:3.1.2'
androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.1'
compile 'com.google.android.gms:play-services-basement:11.8.0'
compile 'com.google.android.gms:play-services-auth:11.8.0'
compile 'com.android.support:recyclerview-v7:26.1.0'
compile 'com.android.support:support-vector-drawable:26.1.0'
}
apply plugin: 'com.google.gms.google-services'