why i can not adding friend code after i remove my old friend code. What should I do?
Unfortunately you can add only one time 1 friend code.
After you delete from your friend list Voucher-Friends you are no longer available to bring him back as voucher friend.
Then the only way to get vouchers is share your friend code to other new players (preferably to get more vouchers)
For infos take a look here: http://forums.flaregames.com/topic/5591-faq-friend-codes-and-vouchers/
Thanks for your understanding.