How to save/load player data?

I want to save what developer products a player has, stats in leaderboard, and any other data a player can have. I have access to api services in studio for my game. I wrote the script at https://developer.roblox.com/en-us/articles/Saving-Player-Data but it didnt work.

The main reason i want to do this to save stages a player has skipped with a developer product. And rebirths too.

Here is my module script in serverstorage:

-- Set up table to return to any script that requires this module script
local PlayerStatManager = {}
 
local DataStoreService = game:GetService("DataStoreService")
local playerData = DataStoreService:GetDataStore("PlayerData")

-- Table to hold player information for the current session
local sessionData = {}
 
local AUTOSAVE_INTERVAL = 60
 
-- Function that other scripts can call to change a player's stats
function PlayerStatManager:ChangeStat(player, statName, value)
	local playerUserId = "Player_" .. player.UserId
	assert(typeof(sessionData[playerUserId][statName]) == typeof(value), "ChangeStat error: types do not match")
	if typeof(sessionData[playerUserId][statName]) == "number" then
		sessionData[playerUserId][statName] = sessionData[playerUserId][statName] + value
	else
		sessionData[playerUserId][statName] = value
	end
end
 
-- Function to add player to the "sessionData" table
local function setupPlayerData(player)
	local playerUserId = "Player_" .. player.UserId
	local data = playerData:GetAsync(playerUserId)
	if data then
		-- Data exists for this player
		sessionData[playerUserId] = data
	else
		-- Data store is working, but no current data for this player
		sessionData[playerUserId] = {Stage=0, Rebirths=0}
	end
end
 
-- Function to save player's data
local function savePlayerData(playerUserId)
	if sessionData[playerUserId] then
		playerData:SetAsync(playerUserId, sessionData[playerUserId])
	end
end
 
-- Function to save player data on exit
local function saveOnExit(player)
	local playerUserId = "Player_" .. player.UserId
	savePlayerData(playerUserId)
end
-- Connect "setupPlayerData()" function to "PlayerAdded" event
game.Players.PlayerAdded:Connect(setupPlayerData)
local function autoSave()
	while wait(AUTOSAVE_INTERVAL) do
		for playerUserId, data in pairs(sessionData) do
			savePlayerData(playerUserId)
		end
	end
end 
return PlayerStatManager

its too long...i know...please help