I don't know how to run the realm example project

If the purpose is to run the Realm example, it would be better to build a copy of the example source code (src/main) after you create a new Android project. Add the bottom part to build.gradle.

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath "io.realm:realm-gradle-plugin:2.0.0"
    }
}

According to the link below, you need to run ./gradlew assembly for the first time to generate the files (including the realm-gradle-plugin of the error screen you uploaded). I think the priority will be to execute the above command first. Please refer to the document for other related information.