Hello Rose1978,
The field to put in the gift certificates during the checkout process is currently not working correctly. The tech team is aware and investigating. You will instead need to redeem it on your account first. You can do this by clicking on My Account at the top of the page. This will take you to a new page where you will see 2 columns of boxes. On the right hand column look for the one titles My Gift Certificates. Clicking on this will take you to a page with a field to redeem the codes, which will turn them into store credit on your account. Once you have done this you can go through the checkout process and select the Use Store Credit option on the payment tab.