error in build.gradle.kts script using extra property

This works for me:

buildscript {
    extra["kotlin_version"] = "1.3.61"

    repositories {
        google()
        jcenter()

    }
    dependencies {
        classpath("com.android.tools.build:gradle:3.5.3")
        classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:${extra["kotlin_version"]}")
        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        google()
        jcenter()

    }
}