The availability of the different form of payments depends on the Point of Sale. Sadly PayPal is not available for Peru.
Try with another credit card, a MasterCard maybe?

Otherwise, make a new reservation. Then select “Presencial / Banca on-line” and then “Tiendas Wong y Metro”. After that, you are going to get a six letter code “Código de Reserva: XXXXXX”, that’s your reservation number. Then call customer service ASAP (cheaper fares have a time-to-live less than 24 hours if they are not issued) and ask for help paying with your card