An weird question!

When I try to use:using UnityEngine.UI;

I got the error message as shown below:

Assets/oliviaclientscript/PanelManager.cs(3,19): error CS0234: The type or namespace name UI' does not exist in the namespace UnityEngine’. Are you missing an assembly reference?

What’s the matter?

What @gif said. You need to download the beta from here to use the UI namespace.