The type or namespace name 'TerrainAPI' does not exist in the namespace 'UnityEngine.Experimental'

Unnecessary using directive. [Assembly-CSharp.Player, Assembly-CSharp]csharp(CS8019) The type or namespace name 'TerrainAPI' does not exist in the namespace 'UnityEngine.Experimental' (are you missing an assembly reference?) [Assembly-CSharp.Player, Assembly-CSharp]csharp(CS0234)

I have no idea why I am facing t$$anonymous$$s error. I am new to unity. Recently I imported "Gaia Pro 2021 terrain system" after successfully Import there was t$$anonymous$$s error:

The type or namespace name 'TerrainAPI' does not exist in the namespace 'UnityEngine.Experimental' (are you missing an assembly reference?) [Assembly-CSharp.Player, Assembly-CSharp]csharp(CS0234)

I went and lookup up the Here It seems I should be able to access it but I am facing an error. Please bear with me, I am new to unity and still trying to figure out heads and tails.

CAN ANYONE HELP ME FIX THIS ISSUE PLEASE