It may be that your payment by credit card is being refused. You will get a message that reads: 

We are unable to complete your payment right now. You might want to retry or to try another payment method.

Often using another credit card also doesn't work. Our payment provider has then rejected your payment for reasons we cannot see due to privacy, but are associated with their security rules. 
However, paying by PayPal does often work. If you don't have a PayPal account; you can very easily open an account with them, it is free and done in under two minutes.
We hope this works for you!