csc in us credit card paypal sandbox code example

Example 1: paypal testing card details

Note: The credit card numbers are applicable only for Payflow users. PayPal 
Payments Pro and Express Checkout users cannot use these test numbers. See the 
Developer Guide (https://developer.paypal.com/docs/payflow/integration-guide/test-transactions/#processors-other-than-paypal)
for correct test credit card numbers.

https://developer.paypal.com/docs/payflow/payflow-pro/payflow-pro-testing/

Example 2: 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