Build WP error.

I tried to build it for WP but received an error message:
"Error building Player: Exception: Error: method `System.Void System.IO.File::WriteAllBytes(System.String,System.Byte[])` doesn't exist in target framework. It is referenced from Assembly-CSharp.dll at System.Boolean AnimationToPNG/<Capture>c__Iterator2::MoveNext().".
How do I fix it!