Okay, we dont need the line ..

but need to tell Android Studio which SDK to use, by clicking somewhere here and also there and then it works!
Easy as pie ..

app/build.gradle

@@ -17,7 +17,6 @@ android {
             proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
         }
     }
-    buildToolsVersion '27.0.3'
 }
 
 dependencies {