There are numerous potential reasons for this.  First, be sure the credit card number you entered was a valid, active account. If you entered it the first time without dashes, try again with the dashes and visa-versa.
Second, if you leave any of the form fields blank like the e-mail address or one of the others, the order will not go through, so take your time and check the form carefully so that all your information is correct. Third, be sure in the form field where it says "billing address" you put the name and address the card is billed to. If the billing address entered and the one on the credit card are different, the processor will assume you are an unauthorized user. This feature is for your protection so someone who may find a lost card cannot use it without knowing the correct address registered with the card. Put the address you would like the item shipped to in the "ship to" form field even if it may be the same as the bill to address.
This system is very secure and will not accept your payment attempt even with a small error. If you are denied please try again and take your time to eliminate errors.