This drove me crazy and took me hours to resolve.

I kept on getting Package [name] does not exist even though I've set up the sbt subproject correctly with aggregate and dependsOn. IntelliJ was happy but play keep on bumming out.

It turns out that when you modify the subproject hierarchy, you would need to terminate the activator run session, and probably best to do a clean, reload and compile before kicking it back on again.

Tested on:
Play Framework 2.4.x
Java