why doesnt this gui work?

Your issue is that when a player clicks the Gui, your function works fine but you don't have anything for it to do!

Debugging

Make sure to debug and read through your scripts, as for yourself you simply misplaced the end) just with a bit of care and time you should've noticed it. If nothing shows on the output then go through the script line by line. I tend to add print("1"), print("2") and so on, so I know where my script is breaking.

Also rather than doing game.ReplicatedStorage, I suggest doing:

local game:GetService("Replicated Storage")

Conclusion

local game:GetService("Replicated Storage")

local screenGui = Instance.new("ScreenGui")
screenGui.Parent = script.Parent

local textButton = Instance.new("TextButton")
textButton.Parent = screenGui
textButton.Position = UDim2.new(0, 300, 0, 650)
textButton.Size = UDim2.new(0, 150, 0, 100)
textButton.BackgroundColor3 = BrickColor.White().Color
textButton.Style = 3
textButton.Text = "builder"

textButton.MouseButton1Down:connect(function()
        if not game.Players.LocalPlayer.Backpack:FindFirstChild("good build") then
            print("Giving "..game.Players.LocalPlayer.Name.." the builder.")
            good = ReplicatedStorage:FindFirstChild("good build"):Clone()
            good = good_build.Parent = game.Players.LocalPlayer.Backpack
        elseif game.Players.LocalPlayer:FindFirstChild("good build") then
            print(game.Players.LocalPlayer.Name.." You dont need it.")
        end
end)