Trying to make a part become random materials when clicking a text button?

As you should know, Materials are now accessed through Enum.Material. And, since Enum is not an actual object, that means Enum (and it's children) are tables.

To sum that up, you don't need to define a materials table, you can just use the Enum;

local materials = Enum.Material

Now, here's the main problem (which is, luckily, easily fixed): The Enum tables are dictionaries, meaning we can't use a math.random. Instead, we will convert the dictionary to an array;

local oMaterials = Enum.Material
local materials  = {}

for _,v in pairs (oMaterials) do
    table.insert(materials,v)
end

As well as that, when searching for a child of a table, do this:

local cName = "val1"
tab[cName]

-- not:
tab.cName

local player = game.Players.LocalPlayer

local oMaterials = Enum.Materials
local Materials  = {}

for _,v in pairs (oMaterials) do
    table.insert(Materials,v)
end

script.Parent.MouseButton1Click:connect(function()
    local PlayersHome = player.PlayersHome
    if PlayersHome.Value ~= nil then        
        local Walls = player.PlayersHome.Value.House.Walls

        for i,v in pairs(Walls:GetChildren()) do
            if v:IsA('UnionOperation') or v:IsA('Part') then
                v.Material = Materials[math.random(1,#Materials)] 
            end
        end
    else
        script.Parent.Parent.Parent.Parent.FailFolder.Fail2.Visible = true
    end
end)

Hope I helped!

~TDP

P.S: Use tab, not space.