Gradle build error: "Plugin with id 'com.google.ar.sceneform.plugin' not found."

You are missing the classpath in the build.gradle (Project) file:

// 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:3.4.1'        
    classpath 'com.google.ar.sceneform:plugin:1.10.0'

    // NOTE: Do not place your application dependencies here; they belong
    // in the individual module build.gradle files
}