For reasons of user security, the Play Store may deem certain purchases out of the ordinary. This automatically prevents unwanted charges on your device. Purely by accident, the system has determined that your purchase was 'high-risk', canceling it in the process.
Don’t worry; the reservation will clear automatically. However, you’ll be unable to make further purchases for 30 minutes (sometimes longer).
In the meantime, verify that your name and billing address are exactly as listed on your credit card.
Unfortunately, we can’t help you here. If you continue receiving the 'high-risk' pop-up, please contact Google for further assistance; they’ll help you out.