For some reason this script says "Attempting to index a nil value", How would I fix this?

The reason that your script broke is that player is not a parameter to the RobloxScriptSignal [event] MouseButton1Click. If this is in a local script, which it should be, because all scripts that work in the PlayerGui, or StarterGui must be LocalScripts due to FilteringEnabled then you would try this method:

local player = game.Players.LocalPlayer
script.Parent.MouseButton1Click:connect(function()
	local lstats = player:WaitForChild('leaderstats')
	if lstats.Tokens.Value >= 60 then
		lstats.Tokens.Value = lstats.Tokens.Value - 60
	else
		script.Parent.Text = 'Insufficient Funds!'
	end
end)

But that probably won't work online because of FIltering Enabled. Your best bet is to use this method of using RemoteFunctions

#This is how to use the recommended method

Step 1: put a RemoteFunction in the ReplicatedStorage named HandleTransaction

Step 2: Put these in your game

Replace your script with this LocalScript and put it in the same place.

local player = game.Players.LocalPlayer
local rf = game.ReplicatedStorage:WaitForChild("HandleTransaction")

script.Parent.MouseButton1Click:Connect(function()
	local transaction = rf:InvokeServer()
	if not transaction then
		script.Parent.Text = 'Insufficient Funds!'
	end
end)

Insert a Script inside the ServerScriptStorage

local rf = game.ReplicatedStorage:WaitForChild("HandleTransaction")

function re.OnServerInvoke(player)
	if player:FindFirstChild("leaderstats") and player.leaderstats:FindFirstChild("Tokens")
then
		if player.leaderstats.Tokens.Value >= 60 then
			player.leaderstats.Tokens.Value = player.leaderstats.Tokens.Value - 60
			-- if you want something to happen as a result of the transaction, like giving the player an item, put the code here.
			return true
		else
			return false
		end
	end
end