yubinglu Thank you for contacting the giffgaff Community.
Please check your debit card is up to date and not expired here https://giffgaff.com/profile/payment-details
You might have some luck if you cleared your browser's data history, please note this deletes saved password, autofill forms, etc, but often this is the only way to resolve Website Accessibility issues with a browser you have been using for a long time. If you don't know how to do this please read the instructions here : https://community.giffgaff.com/d/19483337-how-to-clear-your-browsers-cache-cookies-and-history
Next, I would also delete any stored payment card details from your account. Please go to this link (https://giffgaff.com/profile/payment-details) and manually delete all the fields and click save if prompted.
Now go to purchase a goodybag as normal (https://giffgaff.com/sim-only-plans) or Top-up Credit (https://giffgaff.com/top-up) and manually enter your payment card details. Hopefully, this will sort your problem and don't forget to save your payment card again.
Failing that you'll have to contact an agent who will be able to look into the account for you. Please use this link: (https://support2.giffgaff.com/app/ask/Top-up-balances-and-goodybags/Buying-a-goodybag-or-credit/form/)
Fill out the form as clearly and with as much information as you can.
An agent should reply within 24 hours and you can view their replies in this link: https://giffgaff.com/support/questions