EnumItem cannot be modified?

I'm making this script that has to change the value depending on what random number it lands on.

It has been working without the value part, but I'm not sure how to fix it

It also has an error saying: EnumItem cannot be modified

This is the script:

    local part = math.random(1,100)
	local val = script.Parent.Material
	print(part)
    if part < 41 then
        print("grass")
		val.Value = "grass"
		script.Parent.BrickColor = BrickColor.Green()
    elseif part > 40 and part < 61 then
        print("road")
		val.Value = "road"
		script.Parent.BrickColor = BrickColor.Black()
    elseif part > 60 and part < 101 then
        print("water")
		val.Value = "water"
		script.Parent.BrickColor = BrickColor.new("Bright bluish green")
    end