If you're having trouble making a payment, there are a few things to check:
If You're Logged Into PayPal
If you're logging into PayPal, be sure your credit/debit card which you're using hasn't expired. Just because you have a new card in front of you doesn't mean you've updated the information which PayPal has on file. Update the expiration date and the CVV (3 or 4 digit code on the back of your card) with the new information on PayPal
If You're Not Logged Into PayPal
PayPal doesn't require you to have an account or log in to make a payment. However, if you're trying to pay with a card which is linked to an existing PayPal account, they will not let you use that card. So try a different card or be sure to log into your account to make the payment. I suspect this is a security measure they have so that a lost/stolen card number cannot easily be used on PayPal.