How can I cancel an order for a printable gift card from Amazon

https://www.amazon.ca/gp/help/customer/display.html?nodeId=201937100

"If you accidentally deleted or lost an Amazon.ca Gift Card, please contact us.
You'll need to provide the following information:

• order number (if possible)
• purchaser's name and recipient's name
• physical or e-mail address to which the gift card was sent

We'll contact you by e-mail once we've canceled the original gift card and issued a new one.
If the gift card is for a retailer other than Amazon.ca, please contact that retailer for assistance."

Well you mite trying calling Amazon
About it and give them the bar code
Number. Explain the problem

see if this link helps you out.
amazon uk create account Google Search

locate order in your orders list on Amazon site
press edit order button.
select change payment method.

Note: You need to register gift card on Gift Card section of Your Account before Gift Card can be used as a payment method.

How do I cancel an order for a kindle book - Amazon Kindle

If your order is through Amazon, and the order hasn't shipped yet, there is a possibility that the order can still be cancelled. Go to your orders and if it is not yet shipped, a cancel option may be available.
hope this helps

About Gift Card Redemption Problems

If you're having problems redeeming a gift card to your account, it's possible that the gift card claim code has already been redeemed to your account. After you've entered the claim code on our site, we'll keep the balance in your account for use on future purchases so that you don't need to enter the claim code again. If you're seeing an error message that the gift card has already been redeemed, it's likely because the gift card claim code has already been applied to your account. To check whether the funds are already on your account, view your gift card balance in Your AccountYour Account. If you're still having problems, check the Gift Card Terms & Conditions to make sure your order meets the rules and restrictions for paying with a gift card. If your Amazon.com Gift Card claim code is scratched off or can't be clearly read, you can contact us with the 16 or 30 digit number on the back of the card.If the gift card is for a retailer other than Amazon.com, please contact that retailer for assistance.

http://www.amazon.com/gp/help/customer/display.html?nodeId=468536