unknown property 'LibraryVariants' in build.gradle

The error message, Error: Could not get unknown property 'LibraryVariants' for object of type com.android.build.gradle.LibraryExtension, indicates that this plugin needs to be applied to a module which uses the com.android.application plugin.

So, the simple solution is to only include the google-services plugin in your application module and not your library module:

apply plugin: 'com.android.application'
apply plugin: 'com.google.gms.google-services'

I had the same problem and solved it updating gradle and google-services versions.

// Top-level build file where you can add configuration options common to all sub-projects/modules.
buildscript {
    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.0.0'
        classpath 'com.google.gms:google-services:3.1.1'
        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}