What does this error mean??

"attempt to compare two userdata values" ?????? Output: "18:33:15 -- attempt to compare two userdata values 18:33:19 -- Script 'Players.lucas4114.PlayerGui.ScreenGui.TradeMenu.TradingMenu.Market.Shop.LoadShopUnits', Line 18 18:33:19 -- Stack End" Script:

script.Parent.Changed:connect(function(vis)
	if vis == "Visible" then
		if script.Parent.Visible == true then
			local itemstodelete = script.Parent:GetChildren()
			for i,v in pairs (itemstodelete) do
				if v:IsA("ImageButton") then
					v:Destroy()
				end
			end
			local marketunitswithprice = {}
			local unitval = game.Workspace.SaveGlobalMarket.Items:GetChildren()
			for i,v in pairs (unitval) do
				marketunitswithprice[v.Price.Value] = v
			end
			table.sort(marketunitswithprice)
			for price, marketunit in pairs (marketunitswithprice) do
				local unit = game.ServerStorage.UNITS:FindFirstChild(marketunit.Name)
				local ItemGui = game.ServerStorage.MarketShopItemGui:Clone()
				ItemGui.Name = unit.Name
				ItemGui.Parent = script.Parent
				local ItemGuisinmarket = 0
				for i,v in pairs (script.Parent:GetChildren()) do
					if v:IsA("ImageButton") then
						ItemGuisinmarket = ItemGuisinmarket + 1
					end
				end
				local Ypos = ((ItemGuisinmarket - 1)*130)
				ItemGui.Position = UDim2.new(0,10,0,Ypos +6)
				ItemGui.Frame.ItemInGlobalMarket.Value = marketunit
				ItemGui.Frame.Player.Value = script.Parent.Parent.Parent.Parent.Parent.Parent.Parent.Name
				ItemGui.Frame.ImageLabel.Image = unit.Stats.RespawnGuiImageId.Value
				ItemGui.Frame.UnitDamage.Text = "Damage: "..unit.Stats.Damage.Value
				ItemGui.Frame.UnitDescription.Text = unit.Stats.Description.Value
				ItemGui.Frame.UnitHealth.Text = "Health: "..unit.Humanoid.MaxHealth
				ItemGui.Frame.UnitName.Text = unit.Name
				ItemGui.Frame.UnitPrice.Text = "Price: "..marketunit.Price.Value.." Gold."
				ItemGui.Frame.UnitRange.Text = "Range: "..unit.Stats.Range.Value
				ItemGui.Frame.UnitReloadTime.Text = "Reload Time: "..unit.Stats.ReloadTime.Value
				ItemGui.Frame.UnitRespawnTime.Text = "Respawn Time: "..unit.Stats.RespawnTime.Value
				ItemGui.Frame.UnitSpeed.Text = "Speed: "..unit.Humanoid.WalkSpeed
				ItemGui.Frame.UnitType.Text = unit.Stats.Type.Value
				ItemGui.Frame.BackgroundColor3 = unit.Stats.RespawnGuiBorderColor.Value
				ItemGui.Frame.UnitName.TextStrokeColor3 = unit.Stats.RespawnGuiTextOutlineColor.Value
				ItemGui.Frame.UnitName.TextColor3 = unit.Stats.RespawnGuiTextColor.Value
				if ItemGuisinmarket/2 == math.floor(ItemGuisinmarket/2) then
					script.Parent.CanvasSize = UDim2.new(0,0,(ItemGuisinmarket/2 + 1),0)
				end
			end
		end
	end
end)

You are probably comparing two values such as if you were to compare a position and if you say something like this....

local torso = player.Character.Torso
if torso.Position >= Vector3.new(5,5,5) then
print("Wow...")
end

Now the code above will have an error stating that you're comparing two userdata values but, you could use magnitude for the above example.. So if that is your problem try using magnitude or some other method. Thank you for reading this answer. Hope it helped you out.

As you can see on the error, the problem is in Line 18