Solution:
If you enable Paypal Sandbox option, you need to get a testing developer account (as specified in related settings… see below):
This testing account doesn’t need to be attached with bank and credit card.
That’s why it doesn’t work…
The developer account provide (for testing):
- Admins account (where you will get transactions, notifications, errors)
- Fake customers account for fake purchases.
Once registered your developer account, you will need to create a pair of admin + customers account for one URL shop.