Scope of extra property in a build.gradle.kts script

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1

Hello,
why does the (build.gradle.kts) script below gives the error : Cannot get property ‘kotlinVersion’ on extra properties extension as it does not exist ?

    buildscript {
        extra["kotlinVersion"] = "1.2.70"

        repositories {
            jcenter()
        }

        dependencies {
          classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:${extra["kotlinVersion"]}")
        }
    }
    ```