Errors at checkout tend to occur for the following reasons.
International transactions blocked
Your bank may block online or international purchases if you have not made similar transactions before. You will need to call the bank to confirm that you are indeed making an international purchase online.
Additional verification required
Some banks will send you a text message to confirm your purchase. Check your mobile device for a security code sent from the bank.
Wrong credit card details
Double check your card number, CVV code (3 digits on the back of the card) and address that you entered at checkout.
Insufficient balance or credit
This means that your bank account doesn't have enough funds to cover the cost of the purchase.