Your scalatest dependency is misconfigured. You want scalatest version 2.6 published for Scala 2.12. There is no such combination, hence your build fails. If you have a look at which version of scalatest is available for Scala 2.12, here's the link. As you can see, it's only version 3.0.0. So, you have 3 options (those are changes in your build.sbt file you need to make):

  1. Update your scalatest version to 3.0.0: libraryDependencies += "org.scalatest" %% "scalatest" % "3.0.0" % Test
  2. Downgrade your Scala version to 2.11: scalaVersion := "2.11.8"
  3. Both of the above

I'd say it's a bit too early to use Scala 2.12, since it was only released a couple of days ago, and not all of the dependencies are published for it yet. Scala major versions (2.11 vs 2.12 is a major version upgrade for Scala) are not binary compatible, so you can't use a library compiled with one Scala version in a project that uses the other.

At the same time, I'd go with scalatest 3.0.0 version, since it's the stable one. So all in all, I'd go for option 3 at the moment, even though options 1 and 2 will fix this particular problem, in different ways.

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