How to permanently make this script save tools in your backpack?

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

local ReplicatedStorage = game:GetService('ReplicatedStorage')
local Shop = ReplicatedStorage:WaitForChild('ShopItems')

local function onPlayerJoin(player)  -- Runs when players join

    local playerUserId = "Player_" .. player.UserId  --Gets player ID
    local data = playerData:GetAsync(playerUserId)  --Checks if player has stored data
    if data then
        print('here')
        for k, j in pairs(data) do
            print('loop')
            print(j)
            local giveTool = Shop:WaitForChild(j):Clone()
            giveTool.Parent = player:WaitForChild('Backpack')
        end
    else
        print('error')
        return
    end
end

local function onPlayerExit(player)  --Runs when players exit

    local tool_table = {}

    for _, item in pairs(player.Backpack:GetChildren()) do
        print(item.Name)
        table.insert(tool_table, item.Name)
    end

    local success, err = pcall(function()
        print('Saved')
        local playerUserId = "Player_" .. player.UserId
        playerData:SetAsync(playerUserId, tool_table) --Saves player data
    end)

    if not success then
        warn('Could not save data!')
    end
end

game.Players.PlayerAdded:Connect(onPlayerJoin)
game.Players.PlayerRemoving:Connect(onPlayerExit)