Possible reasons:
1. You don't have at least 1 euro (or its equivalent in your card's currency) on your balance,
2. You use a different card, not MasterCard or Visa,
3. Problem with the bank - bank blocks the attempts to add the card,

4. Card might not be fit for international transactions,

5. Failed 3-D Secure authentication by your bank,
6. Cardholder name or digits are inserted incorrectly,

7. The same card was previously added in another account,
8. Other reasons.

Possible solutions:

1. Make sure you have at least 1 euro (or its equivalent) on your balance,

2. Contact the bank and make sure your card is allowed to make international transactions,

3. Try to add the same card via: Mercuryo web wallet, Mercuryo mobile app, Mercuryo widget.