I cannot check out. I keep being returned to the payment screen. The website will return you to the payment screen if your credit card was declined for any reason. The error should display in red at the top of the page.
The most common reason for credit cards being rejected is an address mismatch. The address that the credit or debit card statements must match the billing address used for the order exactly.
You can change the billing address to using the 'change address' button on the checkout screen.