Adding multiple codes [?, solved!]

The reason your code is not working is because you return true/false within the first if block, meaning your 2nd if block never gets a chance to run. You could just remove the else return false part, or you can merge the if blocks together:

game.ReplicatedStorage.CheckCode.OnServerInvoke = function(player, code)
    if code == "Sub2Tenulty" then -- first code

        local backpack = player:WaitForChild("Backpack")

        local machete = game.ServerStorage["Tenulty's Machete"]:Clone()

        machete.Parent = backpack

        return true
    elseif code == "Sub2Kobe" then --second code

        local backpack = player:WaitForChild("Backpack")

        local machete = game.ServerStorage["Kobe's Machete"]:Clone()

        machete.Parent = backpack

        return true
    else
        return false
    end
end

However, this isn't super efficient. If you had 100 codes and they sent in a wrong code, you'd be going through all 100 codes before you knew that it was wrong. This isn't that big a deal, but we can use tables to keep things efficient:

local codes = {
	Sub2Tenulty = function(player)
		local backpack = player:WaitForChild("Backpack")
	        local machete = game.ServerStorage["Tenulty's Machete"]:Clone()
	        machete.Parent = backpack
	end,
	Sub2Kobe = function(player)
	        local backpack = player:WaitForChild("Backpack")
	        local machete = game.ServerStorage["Kobe's Machete"]:Clone()
        	machete.Parent = backpack
	end,
	["Example of a code with spaces in it"] = function(player)
		-- etc
	end,
}

(Note: You have to use ["This format"] if the code is not a valid variable name. So, if the code starts with anything except a character or _, or if it contains anything besides letters or numbers or _, you need to use it. You can use it for everything if you'd prefer to keep it consistent.)

The idea is that we're storing a mapping between each code and what we want to do when a player enters it. Now to use it:

game.ReplicatedStorage.CheckCode.OnServerInvoke = function(player, code)
	local action = codes[code]
	if not action then -- It wasn't a code
		return false
	end
	-- You could add additional code here to check to see if they've already used it (if you only want to let them use a code once)
	action(player) -- this runs the action associated with the code in the table above, providing the function with the player who sent it in.
	return true
end