I got this Error : invalid argument #3 (string expected, got nil) What is happen?

maybe it is because im using string value on plr.leaderstats.level

local DS = game:GetService("DataStoreService"):GetDataStore("SaveData")
game.Players.PlayerAdded:Connect(function(plr)
	wait()
	local plrkey = "id_"..plr.userId
	local save1 = plr.leaderstats.Uang
	local save2 = plr.leaderstats.Level
	local save3 = plr.Bahan.Daging
	local save4 = plr.Bahan.PenyimpananMax
	local save5 = plr.Bahan.Penyimpanan
	local save6 = plr.Jualan.Sate
	local save7 = plr.Jualan.PenyimpananMax
	local save8 = plr.Jualan.Penyimpanan
	local save9 = plr.Exp.Exp
	local save10 = plr.Exp.LevelUp
	local save11 = plr.Exp.LevelValue
	local save12 = plr.Exp.Point
	local GetSaved = DS:GetAsync(plrkey)
	if GetSaved then
		save1.Value = GetSaved[1]
		save2.Value = GetSaved[2]
		save3.Value = GetSaved[3]
		save4.Value = GetSaved[4]
		save5.Value = GetSaved[5]
		save6.Value = GetSaved[6]
		save7.Value = GetSaved[7]
		save8.Value = GetSaved[8]
		save9.Value = GetSaved[9]
		save10.Value = GetSaved[10]
		save11.Value = GetSaved[11]
		save12.Value = GetSaved[12]
	else
		local NumberForSaving = {save1.Value, save2.Value, save3.Value, save4.Value, save5.Value, save6.Value, save7.Value, save8.Value, save9.Value, save10.Value, save11.Value, save12.Value}
		DS:GetAsync(plrkey, NumberForSaving)
	end
end)

game.Players.PlayerRemoving:Connect(function(plr)
	DS:SetAsync("id_"..plr.userId,game:GetService("HttpService"):JSONEncode{plr.leaderstats.Uang.Value, plr.leaderstats.Level.Value, plr.Bahan.Daging, plr.Bahan.Penyimpanan, plr.Bahan.PenyimpananMax, plr.Jualan.Penyimpanan, plr.Jualan.PenyimpananMax, plr.Jualan.Sate, plr.Exp.Exp, plr.Exp.LevelUp, plr.Exp.LevelValue, plr.Exp.Point})
end)

error ServerScriptService.DataSaving:23: invalid argument #3 (string expected, got nil) i appreciate every helpers