Something went wrong in the Beast lightmapper: File IO error - Could not create process. This is a bug - please report it with a scene reproducing the issue. UnityEditor.DockArea:OnGUI()

when I create a empty new project and a new scene with a simple cube，and set the cube “Static”，I opened lightmapping window clicked “bakescene” button，the error happened： “Something went wrong in the Beast lightmapper: File IO error - Could not create process. This is a bug - please report it with a scene reproducing the issue.

UnityEditor.DockArea:OnGUI()”

My editor ver is 4.6.3