Gui Weapon Shop fixes? [Still UNANSWERD :( ]

After reviewing the code, it looks fine. You just needed to put it in a local script.

--Assuming this is a local script inside the button inside the Gui

local player = game.Players.LocalPlayer
local money = player.leaderstats.KOs
local price = 3
local tool = game.Lighting:FindFirstChild("Knife")

script.Parent.MouseButton1Down:connect(function()
if money.Value >= price then
money.Value = money.Value - price
local a = tool:Clone()
a.Parent = player.Backpack
end
end)

Well, since you haven't gotten an answer yet, I'll try to help. There doesn't appear to be anything wrong with your script, however the problem might be with finding the player, or the knife. Make sure that there's a knife in the Lighting before running the script, and make sure that you're leading it to the correct Parent. To test if those work correctly, do this in a separate script, that has the same parent(Make sure your output window is open):

if player = script.Parent.Parent.Parent.Parent.Parent then
    print(player.Name) --If it prints the correct players name, it works fine.
else
    print("Error")
end

To make sure the knife part is correct, do this:

if game.Lighting:findFirstChild("Knife") then
    if game.Lighting:findFirstChild("Knife"):IsA("Tool") then
        print("Knifes fine")
    else
        print("Error")
    end
else
    print("Error")
end

If either of these come up with an error, then look back at where all the objects are placed, and what you're leading your script to find. This is the best I could do with the info we had, so I hope it helped :P . If not, sorry.