I can't save player data and i don't know what to do?

local PlayerStatManager = {}
 
local DataStoreService = game:GetService("DataStoreService")
local playerData = DataStoreService:GetDataStore("PlayerData")
 
local sessionData = {}
 
local AUTOSAVE_INTERVAL = 60
 
function PlayerStatManager:ChangeStat(player, statName, value)
	assert(typeof(sessionData[playerUserId][statName]) == typeof(value), "ChangeStat error: types do not match")
	local playerUserId = "Player_" .. player.UserId
	if typeof(sessionData[playerUserId][statName]) == "number" then
		sessionData[playerUserId][statName] = sessionData[playerUserId][statName] + value
	else
		sessionData[playerUserId][statName] = value
	end
end
 
local function setupPlayerData(player)
	local playerUserId = "Player_" .. player.UserId
	local success, data = pcall(function()
		return playerData:GetAsync(playerUserId)
	end)
	if success then
		if data then

			sessionData[playerUserId] = data
		else

			sessionData[playerUserId] = {Money=0, Experience=0}
		end
	else
		warn("Cannot access data store for player!")
	end
end

local function savePlayerData(playerUserId)
	if sessionData[playerUserId] then
		local success, err = pcall(function()
			playerData:SetAsync(playerUserId, sessionData[playerUserId])
		end)
		if not success then
			warn("Cannot save data for player!")
		end
	end
end
 

local function saveOnExit(player)
	local playerUserId = "Player_" .. player.UserId
	savePlayerData(playerUserId)
end
 

local function autoSave()
	while wait(AUTOSAVE_INTERVAL) do
		for playerUserId, data in pairs(sessionData) do
			savePlayerData(playerUserId)
		end
	end
end
 

spawn(autoSave)
 

game.Players.PlayerAdded:Connect(setupPlayerData)
 

game.Players.PlayerRemoving:Connect(saveOnExit)
 
return PlayerStatManager