How do you use arguments/variables in a location?

I am trying to make a plugin but it gives an error that says: "Mat" is not a valid Enum material. I have tried parentheses and quotations, parentheses, and quotations. This is the code

local plugin = PluginManager():CreatePlugin()
local toolbar = plugin:CreateToolbar("Filling tools")
local button = toolbar:CreateButton(
    "", 
    "Flood fill", 
    "Potato.png" 
    )

Fillblock=""
active = 0
--____________________________________________

_G.Changeblock = function(Mat)
    Fillblock = Enum.Material.Mat
end
--____________________________________________

button.Click:connect(function()
    if active == 0 then
    plugin:Activate(true)
    active = 1
    print("Welcome to Warman13579's Fill plugin! Type in _G.help() to see the commands")
    wait(1)
    print("Select your material! The list of current materials is: Slate, Brick, WoodPlanks, Sand, Water, and concrete. Type _G.Changeblock(water) (water in quotations) for example.")
    else
    active = 0
    plugin:Activate(false)
    print("Goodbye")
    end
end)
--_____________________________________________

NOTE: Some functions are not implemented like _G.Help()