How can I retrieve missing code from Apple Gift Card?

See what to do: If you can't redeem your Apple Gift Card, App Store Card, or App Store & iTunes Gift Card - Apple Support

Eg: says:

"Code is unreadable

If you can't read the code on the back of your card because it's scratched or damaged, contact Apple Support".

and:

"Contact Apple Support

You might need to provide:

• An image of the full front and back of your card.
• A sales receipt showing when your card was purchased.
• The serial number from your card.

Contact Apple Support"

IMPORTANT: To make sure you provide the correct serial number,

See the above article (scroll to the bottom) for info. and illustrations, to ensure you use the right number.