Welcome, I am Cory. Thank you for asking your question today.
Is it a Kindle, Kobo, Nook, Ect.?
Can I connect remotely and assist you with this?
If you are on a computer right now.
Are you on a computer now?
Because we can not connect remotely, I have found a guide you can use on how to add a card to your account.
Please click - https://www.kobo.com/help/en-US/article/1923/changing-your-credit-card-or-billing-information
Please let me know if this helps you.
Please let me know if you need further help with this.