I need help on this Terrain Decoration Gui Script?

This is my script:

local MainFrame = script.Parent
local HighLow = script.Parent.TextButton



script.Parent.TextButton.MouseButton1Click:Connect(function()
    HighLow.Text = "Low"
    game.Workspace.Terrain.Decoration = false
    script.Parent.TextButton.MouseButton1Click:Connect(function()
        HighLow.Text = "High"
        game.Workspace.Terrain.Decoration = true
    end)
end)

Apparently Decoration isn't a valid member of Terrain.. Can someone help with this??