how to get terrain material to change a value?

so all the code works for regular part materials but i cant get when a player touched a curtain terrain material and no error are showing in output

local mv = script:WaitForChild("Material") or 																								script.Parent.Humanoid:GetPropertyChangedSignal("FloorMaterial")
    mv.Value = tuch.Material.Name or script.Parent.Humanoid.FloorMaterial.Name

	mv.Value == "Mud" then --string value wont change on touch with terriain
		st.SoundId = "rbxassetid://481215525"
	elseif