The module ‘app’ is an Android project without build variants

Above gradle file code seems to be perfect. Probably its nothing to do with app/build.gradle (Module:app). Just open other build.gradle (Project:Android) file in Project window and verify your Android Studio version it must be same as yours.

I replaced from:

dependencies {
    classpath 'com.android.tools.build:gradle:3.2.1'
    // NOTE: Do not place your application dependencies here; they belong
    // in the individual module build.gradle files
}

to my Android Studio v3.0.1 in my case:

dependencies {
    classpath 'com.android.tools.build:gradle:3.0.1'
    // NOTE: Do not place your application dependencies here; they belong
    // in the individual module build.gradle files
}

Press “Try Again” to Sync gradle file. This resolved my problem with a successful build.