I made a GUI (By Local Script) and it's not working, why?

Two things are wrong with this script, one you never parent ScreenGui to the PlayerGui, so it'll never be seen, and two localscripts won't work inside the workspace, except for when they are in the character.

So what you need to do is one Parent this localscript to the StarterGui and two make the script parent ScreenGui

Edit: You are also using UDim2.new() incorrectly. This takes 4 parameters, you are giving it two tables of two numbers. Simply remove {}{} inside these.

local ScreenGui = Instance.new("ScreenGui")
local MAINGUI = Instance.new("Frame",ScreenGui)
local OpenAndClose = Instance.new("TextButton",ScreenGui)
local Button1 = Instance.new("TextButton",MAINGUI)
local Button2 = Instance.new("TextButton",MAINGUI)

--MainGUI

MAINGUI.Position = UDim2.new(0.549, 0,0.129, 0)
MAINGUI.Size = UDim2.new(0, 390,0, 393)
MAINGUI.Visible = false


--Buttons

Button1.Position = UDim2.new(0.563, 0,0.165, 0)
Button1.Size = UDim2.new(0, 200,0, 50)
Button1.Text = "BRICK MATERIAL CHANGE TO NEON"

Button2.Position = UDim2.new(0.563, 0,0.268, 0)
Button2.Size = UDim2.new(0, 200,0, 50)
Button2.Text = "BRICK COLOR CHANGE"

--OpenAndCloseGuiSetUP

OpenAndClose.Position = UDim2.new(0.014, 0,0.041, 0)
OpenAndClose.TextSize = 20
OpenAndClose.Text = "Open/Close Gui"
Open = false
OpenAndClose.MouseButton1Down:connect(function(open)
if Open == false then
    MAINGUI.Visible = true
    Open = true
elseif Open == true then
    MAINGUI.Visible = false
end


--Parent ScreenGui
ScreenGui.Parent=script.Parent --assuming script is in startergui