your sbt build file is not right. your scala version is 2.12.x but you are using libraries compiled in scala version 2.11. use the sbt settings shown below
note: I changed the version of scalatest as 2.x versions are no longer supported for 2.12 version of scala
scalaVersion := "2.12.1"
libraryDependencies ++= Seq(
"org.scala-js" %% "scalajs-test-interface" % "0.6.14",
"org.scalatest" %% "scalatest" % "3.0.1", //version changed as these the only versions supported by 2.12
"com.novocode" % "junit-interface" % "0.11",
"org.scala-lang" % "scala-library" % scalaVersion.value
)
remember to do reload, clean and compile in your sbt console to start clean compile