Getting errors with ScreenOrientation.Portrait

I'm trying to set the screen orientation to portrait for my mobile game and am getting the below error.

error CS0117: `ScreenOrientation' does not contain a definition for `Portrait'

I'm using
to set the orientation to portrait but its not working.

Can anyone tell me what I'm doing wrong? Or how do I prevent the auto-rotation? Its a vertical shooter and I want it to stay in portrait.

Thanks