The error was due to declaring these dependencies in dependencies.gradle:

String scalaVersion = "2.12.15"
dependencySet(group: "org.scala-lang", version: scalaVersion) {
    entry "scala-library"
    entry "scala-reflect"
    entry "scala-compiler"
}

Once I removed the section above and replace it with this section in buid.gradle, error is gone. Thx

String scalaVersion = "2.12.15"
dependencies {
    compileOnly "org.scala-lang:scala-library:$scalaVersion"
    compileOnly "org.scala-lang:scala-reflect:$scalaVersion"
    compileOnly "org.scala-lang:scala-compiler:$scalaVersion"
}