There are several factors here as to why it isn't working. We recommend you to first and foremost to try and clear the cookies on your device. And then to use a different browser to the one you are currently using.
Another tip is to use another device completely and try to make the purchase there, such as on your tablet or your phone if you used your computer previously.
If it still does not work, it could be that your card has a block for online purchases and/or overseas payments. If that is the case, you need to contact your card provider to help you with the issue.
We ask you to try at a later time and contact your bank if the issue persists.