How would I stop this value from being nil?

So I used this guide to learn datastore. I keep getting 13:41:16.994 - ServerScriptService.Stat_Saver:24: attempt to index local 'data' (a nil value)

local DSS = game:GetService("DataStoreService")
local dataStore = DSS:GetDataStore("Stats", "Players")

function makeDataKey(player)
	local ret = "uid_"..player.userId
	return ret
end

function makeDataTable(player)
	local statsTable = {
		AttVal = player:findFirstChild("Data").AttVal.Value
	}
	return statsTable()
end

function savePlayerData(player)
	local key = makeDataKey(player)
	local data = makeDataTable(player)
	dataStore:SetAsync(key,data)
end

function giveDataToPlayer(player, data) 
		--The error is here
		player.Data.AttVal.Value = data.AttVal
end

function loadPlayerData(player)
	local key = makeDataKey(player)
	local data = dataStore:GetAsync(key)
	giveDataToPlayer(player, data)
end

game.Players.PlayerAdded:connect(function(player)
	wait(5)
	loadPlayerData(player)
	print("Loaded Data")
	player.Data.Level.Changed:connect(function()
		savePlayerData(player)
		print("Data saved")
	end)
end)

game.Players.PlayerRemoving:connect(savePlayerData)