Problem with Data Store?

I am making a game and I want to save/load stats using one single string. I use Data Store and String Pattern to save it.

local DS_Stats = game:GetService("DataStoreService"):GetDataStore("Stats")

local patern = ";(%a+):(%w+)" --this means that is search for ";" any letters ":" any letters or numbers"

function addStats(type,path,name,value)
    local stat = Instance.new(type,path)
    stat.Name = name
    stat.Value = value
end

game.Players.PlayerAdded:connect(function (player)
    local DS_key = "user_" .. player.userId
    local Stats = Instance.new("Configuration",player)
    Stats.Name = "Stats"
    local ls = Instance.new("Configuration",player)
    ls.Name = "leaderboard"
    local Stat_String = DS_Stats:GetAsync(DS_key)
    for key, val in Stat_String:gmatch(pattern) do --Problem is comming from this line It says Stat_String is a nil value, it was "for key, val in DS_Stats:GetAsync(DS_key):gmatch(pattern) do" before, but I tried making it a variable. Didn't worked.
        if key == "LT" then
            addStats("StringValue",Stats,"LT",val or "SpawnTown")
            addStats("StringValue",ls,"LT",val or "SpawnTown")
        elseif key == "OCU" then
            addStats("IntValue",Stats,"OCU",val or 0)
            addStats("IntValue",ls,"OCU",val or 0)
        end
    end


end)

game.Players.PlayerRemoving:connect(function(player)
    local key = "user_" .. player.userId
    local stats = player.Stats
    DS_Stats:UpdateAsync(key, function(oldValue)
        local newValue = oldValue or ";LT:SpawnTown;OCU:0"
        newValue = ";LT:"..stats.LT..";OCU"..stats.OCU
        return newValue
    end)
end)

This is what it show on the DevConsole InGame ServerScriptService.OnEnter:20: attempt to index local "Stats_String" (a nil value) Stack Begin Script "ServerScriptService.OnEnter", Line 20 Stack End Content failed because Not Found

I am just Asking why it doesn't work and how to Fix It, Because i don't understand why the script act like the DataStore doesn't exist.