Hi Albert,
For sandbox mode testing you should use credit card details in your account. Go to developer.paypal.com >> login with your account >> Dashboard >> Sandbox Accounts >> Select the paypal pro account >> profile >> Funding >> use the credit card details or use the below credit card details. Then only it will work for Sandbox mode.
Credit card number: 4032031522768281
Credit card type: Visa
Expiration date: 10/2019