Hello Mahmoud Maarek, You will need to open a support case so that we can see why you are not able to create another pool.
Hi, I have unity 300 with code 4.4 and one traditional pool 15 disk 600GB , we need to add 5 disks flash but dynamic pool minimum drives is 6 so we need too create traditional pool by command. /storage / config / create but not working , any solution .      
Create Unity Tradtional pool