How will i recover my missing code of apple gift card

Contact support, they will definitely help. When contacting us, show all receipts and purchase confirmations of your Apple Card.

Did you already have a look at this related article and the troubleshooting mentioned?

If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

Contact the official Apple Support, you can use this Online link as a start:

Get Apple Gift Card Support