Generally this is because VISA, our credit card partner does not recognise the card details you have provided or there is some data missing. Please check that you have entered all details correctly, especially the long number, expiry date, CVV security number (the last three digits on the back of the card) and on your contact details that you have put in the correct post code for the address that the card is registered with.
If everything has been entered correctly and we still cannot authorise the transaction, please contact your credit or debit card company direct.