Why is the error Conflicting cross-version suffixes?

The conflicts appear because:

1. you've specified your Scala version to be 2.11
2. you've explicitly specified the Scala version (2.10) for the reactivemongo and poi-scala libraries.

The fix is to use the %% operator for those two libraries as well.

"org.reactivemongo" %% "reactivemongo" % reactiveMongoVersion,
"info.folone"       %% "poi-scala"     % foloneVersion

That's the purpose of the %% operator. To append the declared Scala version (2.11 in your case) to the artifact name.

I had the same problem and I simply removed the scalaVersion tag from my sbt file and modified the line

libraryDependencies += "org.apache.spark" %% "spark-core" % "1.6.0"

to

libraryDependencies += "org.apache.spark" % "spark-core_2.11" % "1.6.0"

and the problem went away.

I have tried using %% but it didn't work. I have manually excluded it using

("org.reactivemongo" % "reactivemongo" % reactiveMongoVersion)
    .exclude("com.typesafe.akka", "akka-actor_2.10")
    .exclude("org.scalaz", "scalaz-effect")
    .exclude("org.scalaz", "scalaz-core")