Gradle sync failed: Cause: compileSdkVersion is not specified

You are using android support library of 27.+ so you will have to give sdk version 27 as compileSdkVersion and targetSdkVersion otherwise your project does not know for which platform your project should be built. These parameter should be given in android directory like this in build.gradle(app):

android {
    compileSdkVersion 27
    buildToolsVersion '27.0.3'
    defaultConfig {
        applicationId "com.example.abc.test"
        minSdkVersion 21
        targetSdkVersion 27
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

Just paste this code below apply plugin: 'com.android.application' this line