A "SECURITY VIOLATION" is a declined transaction error message back from the payment gateway, and is actually really easy to fix!
1. Go to the gateways section of your account by clicking "Gateways" along the top of the page.
2. Click "Edit" next to the payment gateway.
3. Verify you have the exactly correct Merchant ID & Merchant Key from your account acceptance email. The most commonly confused characters are "1" with "I" & "0" with "O". Usually best to just copy and paste.