Smart Subscriptions payment plugins are running in "sandbox" mode. If you choose PayPal as payment method you have to use the following "demo" credit card

Country Spain
Type Visa
Card No. 4434645682646633 
Expiration 10/20 (actually any future date will work)
CVV 123 (actually any code will work)

You will also have to provide a valid address and telephone. If you are not familiar with spanish regions, you can use the following ficticious data:

Address C/Tomas Edison, 2
Zip code 08860
City Castelldefels
Province Barcelona
Phone 931821234