There are multiple possible reasons for encountering this error; however, your payment provider doesn't always transmit the exact reason to us.
The majority of cases occur from the fact that we are based in France. Some international payments may fail because your bank flags the attempted payment as suspicious (as we are based far from your location where you probably make most of your transactions). We advise you to contact your bank and confirm that you tried to make this payment. Your bank will then authorize the payment and you'll be able to subscribe to the plan of your choice.