The reason for this is most likely that the credit card you selected as default on your PayPal account, could not be used and the transaction could not be authorised. You need to contact PayPal customer service team to discuss the issues relating to the selected card and how to resolve it. The customer service team will help you understand why the payment was denied.
For data protection reasons PayPal won't be able to discuss details relating to your PayPal account with us.
Meanwhile you may want perhaps to use other financial instruments in your funding matrix, for example you can select an alternative such as a different card, a bank account and even a balance if it is available on you PayPal account.