Unity3d中报错‘SpatialTracking‘ does not exist in the namespace ‘UnityEngine‘的解决办法

报错原文

The type or namespace name 'SpatialTracking' does not exist 
in the namespace 'UnityEngine' (are you missing an assembly reference?)

解决方法

打开包体管理器：Windows> Package Manager

找到ARFoundation 并安装：