why is there and error on line 33 something to do with "FindFirstChild"?

Seems like you're trying to only set Points if leaderstats exists, which is not what your code is currently doing. Looks like you have some confusion in general about if blocks. Change the MouseButton function to something like this:

script.Parent.MouseButton1Click:Connect(function()
    local leaderstats = player:FindFirstChild("leaderstats")
    if leaderstats then
        local Points = leaderstats:FindFirstChild("Points")
        if Points then
            local hasChocolateMilk = checkForChocolateMilk()
            if Points.Value >= Cost and hasChocolateMilk == false then
                Points.Value = Points.Value - Cost
                local ChocolateMilk = game.ServerStorage.ChocolateMilk:Clone()
                ChocolateMilk.Parent = player.Backpack
            end
            local found = false
            for i, v in pairs(player.Backpack:GetChildren()) do
                if v.Name == "ChocolateMilk" then
                    found = true
                    break
                end
            end
            if found == true then
                script.Parent.Parent.Parent.ChocolateMilkOwned.Visible = true
                script.Parent.Roundify.ImageColor3 = Color3.fromRGB(255, 44, 47)
            end
        end
    end
end)

You also should break out of the for look when what you want is found, as seen above

In your code, you check if leaderstats has been created and then create the Points IntValue, however directly after it you check for points outside of the statement, therefore nullifying the purpose of the if statement to catch the potential error.

tl;dr: remove line 33