Hi, @knorman !
Are you trying to pay for your Shopify subscription or is this for a customer trying to place an order through you're checkout?
If you're trying to pay a Shopify invoice, the credit card must be a VISA, Mastercard, or AMEX. Hybrid debit/credit cards for these brands are supported; Prepaid cards are not supported although they may sometimes work. Virtual credit cards are also not accepted.
If this is for a customer, generally they need to register an address or verify the card. Cards will usually come with instructions that explain how to set this up. This usually occurs when the customer hasn't verified their card in this way.