First of all, do not insist. You risk making the purchase several times. Then, contact the support team and describe precisely your problem, the possible error message you are receiving, and especially at what stage. They will be able to investigate what is wrong.
In most cases, banks now ask you to confirm your transaction via a temporary code sent by sms on your cell phone or by a confirmation to be done on your bank's application on your smartphone. This is commonly known as double authentication or 3D-secure. It is specific to each bank, so we invite you to get in touch with your bank to find out what security it has put in place for your payments on the Internet.
If the error message says "transaction refused by provider", it means that our payment provider is blocking your transaction, probably because you are trying to pay from a foreign country. In this case, unfortunately, we advise you to make your purchase via another payment method.