I can't save money data and I don't know why?

You should save using tables. This will make it easier in the future to add other values too. I fixed up your code below but keep in mind this is untested.

local Players = game:GetService("Players")
local DataStoreService = game:GetService("DataStoreService")
local DataStore = DataStoreService:GetDataStore("-xBuckDatax-")

-- Making the saving inside a function it shortened your code by a bit.
local function Save(player)
    local key = "Player_"..player.UserId

    local save = {
        ["ObbyBucks"] = player.leaderstats["ObbyBucks<|>"].Value
    }

    --saves their key and money to the data store
    local success, err = pcall(function()
        DataStore:SetAsync(key, save)
    end)

    if not success then
        warn("Failed to over-write data"..tostring(err))
        return
    end
end

-- I made this a function so it can be called whenever
local function Load(player)
    -- saves the player Id as key
    local key = player.UserId

    local folder = Instance.new("Folder")
    folder.Name = "leaderstats"
    folder.Parent = player -- setting the parent in the .new is deprecated

    local money = Instance.new("IntValue")
    money.Name = "ObbyBucks<|>"
    money.Parent = folder

    local moneyAlready

    local success, err = pcall(function()
        moneyAlready.Value = DataStore:GetAsync(key)
    end)

    if not success then
        warn("Failed to read data"..tostring(err))
        return
    end

    if moneyAlready then
        money.Value = moneyAlready.ObbyBucks
    else
        Save(player)
    end
end

Players.PlayerAdded:Connect(Load)
Players.PlayerRemoving:Connect(Save)

All I did was improve the code.

What I did:

Split the DataStore variables, made the saving and loading in a function that you control when it's called, editing the save and load system, removed deprecated code.

More information on DataStores