For the sandbox / testing, your life is somewhat easier if you create a sandbox PayPal account. Otherwise, use "... or Pay with Debit or Credit Card" / AmEx 378282246310005 , CVV 1234, something close to a real address, a fake phone number, continue as guest...

Note that you may have to log out of various PayPal accounts when you use your real developer account and then confuse a sandbox user login.