Bear in mind that PayPal will not let you "pay yourself", so you can't test PayPal checkout with the same PayPal account that would receive the payment, and the option to Pay with Credit or Debit Card on PayPal's checkout site will not work with a card that's registered to the same person as the receiving PayPal account, nor with any card that's already linked to any PayPal account.
You can only test a live PayPal checkout with someone else's PayPal account or card that isn't linked to any PayPal account -- e.g., borrow a friend or colleague's PayPal account, or just have them perform the test checkout for you.