attempt to perform arithmetic (add) on Instance and number? (line 13)

local HolderBG = script.Parent.HolderBG
local Holder = script.Parent.HolderBG.Holder
local RedeemBG = script.Parent.HolderBG.Holder.RedeemBG
local RedeemButton = script.Parent.HolderBG.Holder.RedeemBG.Redeem
local CodeBox = script.Parent.HolderBG.Holder.CodeBox
local CloseButton = script.Parent.HolderBG.Holder.Close
local EarningsButton = script.Parent.HolderBG.Holder.Earnings
local SSS = game:GetService("ServerScriptService")
local player = game.Players.LocalPlayer

RedeemButton.MouseButton1Click:Connect(function()
	local Code = CodeBox.Text
	local Redeemed = game.ReplicatedStorage.CodeGuiEvents.RedeemCode:InvokeServer(Code)
	
	if Redeemed == true then
		print(player.Name.." redeemed the "..CodeBox.Text.." code!")
	end
end)

game.ReplicatedStorage.CodeGuiEvents.FeedbackEvent.OnClientEvent:Connect(function(Message)
	CodeBox.Text = Message
end)

while wait() do
	if string.find(CodeBox.Text,"%s+") then
		CodeBox.TextColor3 = Color3.fromRGB(0, 0, 0)
	end
end

Your error doesn't seem to pertain to this script. The fact that you tried to get ServerScriptService should have errored before line 13 anyways.

-- You can't access ServerScriptService from a LocalScript
--local ServerScriptService = game:GetService("ServerScriptService")
local Players = game:GetService("Players")
local ReplicatedStorage = game:GetService("ReplicatedStorage")
local player = Players.LocalPlayer

local HolderBG = script.Parent:WaitForChild("HolderBG")
local Holder = HolderBG.Holder
local RedeemBG = Holder.RedeemBG
local RedeemButton = RedeemBG.Redeem
local CodeBox = Holder.CodeBox
local CloseButton = Holder.Close
local EarningsButton = Holder.Earnings

RedeemButton.MouseButton1Click:Connect(function()
	local Code = CodeBox.Text
	-- Asking the server a question can cause an infinite yield/hang the server, use a remote event instead.
	ReplicatedStorage.CodeGuiEvents.FeedbackEvent:FireServer(Code)
end)

ReplicatedStorage.CodeGuiEvents.FeedbackEvent.OnClientEvent:Connect(function(Message)
	CodeBox.Text = Message
end)

--> If you can avoid using a loop, do it. 
CodeBox:GetPropertyChangedSignal("Text"):Connect(function()
	if string.find(CodeBox.Text,"%s+") then
		CodeBox.TextColor3 = Color3.new(0, 0, 0)
	else
		-- ?
	end
end)

--[[ Server RemoteEvent instead of RemoteFunction
ReplicatedStorage.CodeGuiEvents.FeedbackEvent.OnServerEvent:Connect(function(Player, Code)
    --> if not player or not code or not code is a string then assert error
    assert ( Player and Code and typeof(Code) == 'string',
        not Code and 'No code provided',
        or typeof(Code) ~= 'string' and 'Code needs to be a string'
        or 'No Player provided?!')
    if code is real and player hasn't reedeemed code then 
        ReplicatedStorage.CodeGuiEvents.FeedbackEvent:FireClient (Player, ('%s redeemed!'):format(Code) )
        -- > Do something for the code
    else
        ReplicatedStorage.CodeGuiEvents.FeedbackEvent:FireClient (Player, ('%s was not redeemed'):format(Code) )
    end
end)
--]]