Hello @AYTS,
Thanks for sharing your question with the Klaviyo Community!
A payment being declined can occur for a variety of reason. From my experience, the most common cause of this is due to either your billing address not matching or the bank/card provider has declined the payment for some reason. I would suggest checking to make sure your billing address entered correctly matches what you have listed for the card. If it does, then I would contact your credit card provider for more details as to why the payment was declined. Once cleared up, you can try to enter your card information again!
For more information on payments and how Klaviyo charges, I would suggest taking a look at the How Klaviyo Bills Customers for Emails and How Klaviyo Bills Customers for SMS Help Center articles.
I hope this helps!
David