Error in Synchronizing SBT and IntelliJ IDEA projects

[installing scala test libraryDependencies error]

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.