I encountered following error while I was installing a fresh Play development environment in to my work laptop:

C:\Work\Java\workspace\myapp>activator run
Getting Scala 2.10.4 (for sbt)...
:: retrieving :: org.scala-sbt#boot-scala
        confs: [default]
        0 artifacts copied, 5 already retrieved (0kB/31ms)
Error: Could not retrieve Scala 2.10.4: missing scala.tools.nsc.Global

The suggested fix of this SO post helped me out. Advice in the post was for Ubuntu, but in Windows, the .sbt is located in your user/home directory too and removing the .sbt\boot directory got rid of the error message.

 C:\Users\<username>\.sbt\rd /s boot