This kata should be 5 kyu if random tests might generate empty strings as random words:

private def randomWord(): String =
  Random.alphanumeric.filter(_.isLetter).take(Random.nextInt(20)).mkString.toLowerCase

And this would work as expected (no random spaces before/between/after words):

private def randomWord(): String =
  Random.alphanumeric.filter(_.isLetter).take(Random.nextInt(20) + 1).mkString.toLowerCase