I have a saving script, but why is it not saving?

-- Setup table that we will return to scripts that require the ModuleScript.
local PlayerStatManager = {}

-- Create variable for the DataStore.
local DataStoreService = game:GetService("DataStoreService")
local playerData = DataStoreService:GetDataStore("PlayerData")

-- Create variable to configure how often the game autosaves the player data.
local AUTOSAVE_INTERVAL = 5

-- Number of times we can retry accessing a DataStore before we give up and create
-- an error.
local DATASTORE_RETRIES = 3

-- Table to hold all of the player information for the current session.
local sessionData = {}

-- Function the other scripts in our game can call to change a player's stats. This
-- function is stored in the returned table so external scripts can use it.
function PlayerStatManager:ChangeStat(player, statName, changeValue)
    sessionData[player][statName] = sessionData[player][statName] + changeValue
end

-- Function to retry the passed in function several times. If the passed in function
-- is unable to be run then this function returns false and creates an error.
local function dataStoreRetry(dataStoreFunction)
    local tries = 0 
    local success = true
    local data = nil
    repeat
        tries = tries + 1
        success = pcall(function() data = dataStoreFunction() end)
        if not success then wait(1) end
    until tries == DATASTORE_RETRIES or success
    if not success then
        error("Could not access DataStore! Warn players that their data might not get save")
    end
    return success, data
end

-- Function to retrieve player's data from the DataStore.
local function getPlayerData(player)
    return dataStoreRetry(function()
        return playerData:GetAsync(player.UserId)
    end)
end

-- Function to save player's data to the DataStore.
local function savePlayerData(player)
    if sessionData[player] then
        return dataStoreRetry(function()
            return playerData:SetAsync(player.UserId, sessionData[player])
        end)
    end
end

-- Function to add player to the sessionData table. First check if the player has
-- data in the DataStore. If so, we'll use that. If not, we'll add the player to
-- the DataStore.
local function setupPlayerData(player)
    local success, data = getPlayerData(player)
    if not success then
        -- Could not access DataStore, set session data for player to false.
        sessionData[player] = false
    else
        if not data then
            -- DataStores are working, but no data for this player
            sessionData[player] = {Money = 0, Rebirths = 0}
            savePlayerData(player)
        else
            -- DataStores are working and we got data for this player
            sessionData[player] = data
        end
    end 
end

-- Function to run in the background to periodically save player's data.
local function autosave()
    while wait(AUTOSAVE_INTERVAL) do
        for player, data in pairs(sessionData) do
            savePlayerData(player)
        end
    end
end

-- Bind setupPlayerData to PlayerAdded to call it when player joins.
game.Players.PlayerAdded:connect(setupPlayerData)

-- Call savePlayerData on PlayerRemoving to save player data when they leave.
-- Also delete the player from the sessionData, as the player isn't in-game anymore.
game.Players.PlayerRemoving:connect(function(player)
    savePlayerData(player)
    sessionData[player] = nil
end)

-- Start running autosave function in the background.
spawn(autosave)

-- Return the PlayerStatManager table to external scripts can access it.
return PlayerStatManager

This is a module script. I copied it from the Roblox website, please help I'm confused why it isn't saving I already have a leaderboard that sets up the two values.