If you get error of :
"your card was declined. try a different card. paypal sandbox"

As Ciaran suggested, either use a card number provided by a random card 
generator tool or use one of the card numbers provided on the first thread in 
this forum.
https://www.paypal-community.com/t5/Sandbox-Environment/Testing-Card-Payments-Within-The-Sandbox-Env

reference : https://www.paypal-community.com/t5/Sandbox-Environment/Unable-to-add-card/td-p/1277274#
Some solution to this issue:
https://www.paypal-community.com/t5/Sandbox-Environment/Testing-Card-Payments-Within-The-Sandbox-Environment/m-p/1253314#M186