There are several reasons why a payment might be unsuccessful. One of the most common reasons is that your bank or credit card issuer may automatically decline the transaction, suspecting it to be fraudulent. This often occurs when attempting to make a payment abroad. A possible reason for this could be that some required information, such as the billing address, is mandatory in your region but not necessarily in Europe. Alternatively, it could simply be because the payment is international.
You can most likely fix this issue by contacting your credit card issuer / bank and asking them to allow all payments going abroad.
Another possible reason might be that, unfortunately, some of the banks / credit card companies outside of the EU do not use 3DS-countersignature and this is the reason why payment can not go through normally.
If this is the reason, this is nothing we can change. It is the EU directive PSD2.
The only options in that case would be to try another payment method or payment card.
If they payment was reserved from your bank account balance, but you didn't receive an automated confirmation email, most likely the payment failed. In these cases the reservation will automatically be lifted usually within few days according to your bank or credit card issuer policies.