Trying to save a folder inside a player, what's wrong?

Hi. I have a folder inside a player (not the character) and I want to save it in a datastore. However, it doesn't seem to work. I'm highly confused, any help?

local DS = game:GetService("DataStoreService"):GetDataStore("Items")
game.Players.PlayerAdded:Connect(function(plr)
	wait()
	local folder = Instance.new("Folder")
	folder.Parent = plr
	folder.Name = "Folderitems"
	local plrkey = "id2_"..plr.userId
	local savevalue = folder:GetChildren()
	
	local GetSaved = DS:GetAsync(plrkey)
	if GetSaved then
		local input = GetSaved[1]
		local values = input:split("|")
for i = 1, #values do
	local number = Instance.new("NumberValue")
	number.Name = values[i]
	number.Parent = folder
end
	else
		local NumbersForSaving = {savevalue}
		DS:GetAsync(plrkey, NumbersForSaving)
	end
end)

game.Players.PlayerRemoving:Connect(function(plr)
	local savevalue = plr.Folderitems:GetChildren()
		local concat = table.concat(savevalue,"|")
	DS:SetAsync("id2_"..plr.userId, {concat})
end)

Right now I'm getting this error: ServerScriptService.DataStore2:27: invalid value (userdata) at index 1 in table for 'concat'

As im a little busy i cant help much but make sure api services are on and use this (this is a script from my game i didnt edit it as i dont want to screw it up as i also dont get data stores)

local DataStoreService = game:GetService("DataStoreService")
local PopularityDataStore = DataStoreService:GetDataStore("PopularityDataStoreExtremeAlpha")
local PopularityXPDataStore = DataStoreService:GetDataStore("PopularityXPDataStoreExtremeAlpha")

game.Players.PlayerAdded:Connect(function(Player)
	
	--  // Stats Folder
	local Stats = Instance.new("Folder")
	Stats.Name = "Stats"
	Stats.Parent = Player
	
	--  // PopularityLevel
	
	local PopularityLevel = Instance.new("IntValue")
	PopularityLevel.Parent = Stats
	PopularityLevel.Name = "Popularity"
	
	--  // PopularityXP
	
	local PopularityXP = Instance.new("IntValue")
	PopularityXP.Parent = Stats
	PopularityXP.Name = "PopularityXP"
	
	--  // Load Data
	
	--  // LvlData
	
	local LvlData
	local success, errormessage = pcall(function()
		LvlData = PopularityDataStore:GetAsync(Player.UserId.."-PopularityLvl")
	end)
	
	--  // XPData
	
	local XPData
	
	local success, errormessage = pcall(function()
		XPData = PopularityXPDataStore:GetAsync(Player.UserId.."-PopularityXP")
	
	if success then
		PopularityLevel.Value = LvlData
		PopularityXP.Value = XPData
	end
end)
	
game.Players.PlayerRemoving:Connect(function(Player2)
	
	--  // Save Data
	
	--  // PopularityLvl
	
	local success, errormessage = pcall(function()

		--  // PopularityXP

		PopularityDataStore:SetAsync(Player2.UserId.."-PopularityXP",Player2.Stats.PopularityLvl.Value)

		--  // PopularityLvl

		PopularityDataStore:SetAsync(Player2.UserId.."-PopularityLvl",Player2.Stats.PopularityXP.Value)
		
		--  // End
		end)	
	end)	
end)