It should be dependsOn
not depends
.
You should also point to both projects from the folder root:
name := "java-test"
version := "1.0-SNAPSHOT"
playJavaSettings
lazy val mainProject = project.in(file("."))
.aggregate(subProject, playProject)
.dependsOn(subProject, playProject)
lazy val subProject = project.in(file("sub-projects/sub-project-1"))
//play project depends on subProject...
lazy val playProject = project.in(file("play-project")).dependsOn(subProject)