When testing the card checkout features on sandbox its advisable to use test card numbers The sandbox pretty much accepts anything that passes the Luhn Algorithm test (See here).
You can generate card numbers using external sites like these:
Or you can generate them within PayPal using this guide below:
I'd also generally recommend not to re-use the same card number for a large amount of transactions
Rotate the card numbers you use for testing purposes to ensure a smooth experience
I found https://creditcard.run/_Credit Card Generator tool and generator creating this fake numbers.
VISA Fake Numbers:
448516598580595-7, 453254046516095-4, 448525019433182-0
Amex Fake Numbers:
37552791891041-4, 37928800574157-8, 37536394661162-0
Mastercard Fake Numbers:
555751493175315-2, 222655761699167-8, 527735993203017-9
Usually paypal accepts, but sometimes paypal not accept this numbers.
Yep, these are for use on the sandbox environment and are completely safe to use, basically as long as the card number passes the Lunh Algorithm test we'll accept it on sandbox.