Possible reasons:
You use a different card, not MasterCard or Visa,
You don't have at least 1 EUR (or its equivalent in your card's currency) on your balance,
Problem with the bank - bank blocks the attempts to add the card,
Card might not be fit for international transactions,
Failed 3-D Secure authentication by your bank,
Cardholder name or digits are inserted incorrectly,
The same card was previously added in another account,
Other reasons.
Possible solutions:
1. Make sure you have at least 1 EUR (or its equivalent) on your balance,
2. Contact the bank and make sure your card is allowed to make international transactions,
3. Make sure your card complies with the mandatory 3DS standard,
4. Contact our support team.