There's usually two super simple reasons why this could be;
Firstly, are you seeing this message?
This limit is placed on all accounts by default to ensure our platform isn't used for illegal reasons such as fraud and spam. Simply get in touch with our customer care team and we'll be able to sort you out.
If not, then it's likely your credit card has been declined. Check that you've typed in your card number, CSV (the security number on the back of your card) and expiry date correctly and that your card hasn't gone over its limit.