Please confirm first if you have registered and used PayPal before. If so, here are some solutions: 

‘problem was with paypal it had an old credit card number and wouldn't allow customer to use theirs new one, even when saying customer wanted to continue as Guest, without first removing the "expired" one. Customers can not had this problem before and basically had to reset everything on PayPal. Just something customer that don't use PayPal on a regular basis need to be aware of’.