What do I do to save the player's leaderstat?

I do not really understand the saving system. I tried different things to make it save a leaderstat, but it did not work. The output say error because the leaderstat did not save. At the bottom of the script, I tried PlayerStatManager:ChangeStat() because I thought it would save the player's leaderstat. What do I do to save the player's leaderstat? I tried many different things, but it did not work, so I need your help. Thanks!

local PlayerStatManager = {}

local DataStoreService = game:GetService("DataStoreService")
local playerData = DataStoreService:GetDataStore("PlayerData")

local AUTOSAVE_INTERVAL = 64

local DATASTORE_RETRIES = 3

local sessionData = {}

function PlayerStatManager:ChangeStat(player, statName)
    sessionData[player][statName] = sessionData[player][statName]
end

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()
    end
    return success, data
end

local function getPlayerData(player)
    return dataStoreRetry(function()
        return playerData:GetAsync(player.UserId)
    end)
end

local function savePlayerData(player)
    if sessionData[player] then
        return dataStoreRetry(function()
            return playerData:SetAsync(player.UserId, sessionData[player])
        end)
    end
end

local function setupPlayerData(player)
    local success, data = getPlayerData(player)
    if not success then
        sessionData[player] = false
    else
        if not data then
            sessionData[player] = {Level = 1}
            savePlayerData(player)
        else
            sessionData[player] = data
        end
    end
end

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

game.Players.PlayerAdded:connect(setupPlayerData)

game.Players.PlayerRemoving:connect(function(player)
    savePlayerData(player)
    sessionData[player] = nil
end)

spawn(autosave)

while true do
    for _, player in pairs(game.Players:GetPlayers()) do
        PlayerStatManager:ChangeStat(player, player.leaderstats.Level.Value)
    end
    wait(64)
end