To use ammonite as a shell you must(!) setup a configuration file
~/.ammonite/predef.sc with a least the following minimal content:

interp.load.ivy(
  "com.lihaoyi" %% "ammonite-ops" % ammonite.Constants.version
)
interp.load.ivy(
  "com.lihaoyi" %
  s"ammonite-shell_${scala.util.Properties.versionNumberString}" %
  ammonite.Constants.version
)
// This @ is necessary for Ammonite to process the `interp.load.ivy`
// before continuing.
@
val shellSession = ammonite.shell.ShellSession()
import shellSession._
import ammonite.ops._
import ammonite.shell._
ammonite.shell.Configure(interp, repl, wd)