Build error

Hello. Thanks for your nice work.
I got trouble with building my project after I imported this project as a module.
The error message I got is 'Caused by: groovy.lang.MissingPropertyException: Could not get unknown property 'compileSdkVersion' for root project 'My Project' of type org.gradle.api.Project.'.

my 'build.gradle (:app)' is as below:

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

android {
compileSdkVersion 28
buildToolsVersion '28.0.3'

defaultConfig {
applicationId "com.example.myaudioplayer"
minSdkVersion 23
targetSdkVersion 28
versionCode 1
versionName "1.0"

testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
}

buildTypes {
release {
minifyEnabled false
proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
}
}

}

dependencies {
implementation fileTree(dir: 'libs', include: ['*.jar'])
implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
implementation 'androidx.appcompat:appcompat:1.1.0'
implementation 'androidx.core:core-ktx:1.2.0'
implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
testImplementation 'junit:junit:4.12'
androidTestImplementation 'androidx.test.ext:junit:1.1.1'
androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'

api 'com.theartofdev.edmodo:android-image-cropper:2.8.+'
}`

Is there anything I missed?