Why does Unity not recognize GameCenterPlatform?

So, I was looking into how to have GameCenter show the achievement banner when the user completes an achievement and I found this in the documentation: GameCenterPlatform in Unity Scripting Reference

However, when I use "GameCenterPlatform", Unity does not recognize it and I get the following error: Unknown identifier: 'GameCenterPlatform'.

I continue to get this error when using the code copied directly from the documentation (Also: I am working in JavaScript).

Does anyone know why this is happening?

Thanks.