Exception in thread "main" java.lang.NoClassDefFoundError: scala/collection/mutable/SynchronizedBuffer$class

_2.10 in the Spark dependency means it works with Scala 2.10, not 2.12. As you can see here, there is no spark-core_2.12, so use scalaVersion := "2.11.8" and "org.apache.spark" %% "spark-core" % "2.1.0". Read more at http://www.scala-sbt.org/0.13/docs/Cross-Build.html.