The title is to long i'll put it in the descriptinon?

You inversed the places for the ends of each code block.

Correct version:

local player = game.Players.LocalPlayer
local PlayerGui = player:WaitForChild("PlayerGui")

if script.Parent.Text == ("BETAISRELEASED") then 
    PlayerGui.Menu.MainMenu.CreditsFrame.RedeemCode.MouseButton1Click:Connect(function() 
PlayerGui.Menu.MainMenu.CreditsFrame.RedeemCode.Text = '("Succesfully Redeemed!") '
wait(2) 
PlayerGui.Menu.MainMenu.CreditsFrame.RedeemCode.Text = '("Redeem")'
end)
end

you actually need to do player.PlayerGui b/c all the GUIs in StarterGui are cloned into the Player.PlayerGui for every player that joins.. all visible GUIs in Player.PlayerGui will display on the player's screen

The problem is you didn't close off the function call: jhere is your code rewritten:

local player = game.Players.LocalPlayer
local PlayerGui = player.PlayerGui

if script.Parent.Text == ("BETAISRELEASED") then 
PlayerGui.Menu.MainMenu.CreditsFrame.RedeemCode.MouseButton1Click:Connect(function() 
PlayerGui.Menu.MainMenu.CreditsFrame.RedeemCode.Text = ("Succesfully Redeemed!") 
wait(2) 
PlayerGui.Menu.MainMenu.CreditsFrame.RedeemCode.Text = ("Redeem") 
end
end)

if script.Parent.Text == ("BETAISRELEASED") then 
game.StarterGui.Menu.MainMenu.CreditsFrame.RedeemCode.MouseButton1Click:Connect(function() 
game.StarterGui.Menu.MainMenu.CreditsFrame.RedeemCode.Text = ("Succesfully Redeemed!") 
wait(2) 
game.StarterGui.Menu.MainMenu.CreditsFrame.RedeemCode.Text = ("Redeem") 
end)
end

you forgot to close the function lol also read the answer above. It is also useful. It's cloned into the player's PlayerGUI so that script might not work if you don't set it as game.Players.LocalPlayer.PlayerGUI.Menu

edit: sorry made a mistake script fixed