Apple Gift certificate

Hello I received an apple gift certificate on July 23 after I returned my old iPhone. I just wanted to order a new product however the gift certificate number starting with GCA doesn't work, I keep getting the error 'invalid'. I can't find any other number (apparently should start with X) and unable to place an order. Can you please let me know what the correct number is to be filled in into the utiliser gift card bracket during checkout?