There are a few typical scenarios that can cause that.
You may have insufficient funds on your account
You may have entered the CVC code incorrectly more than 3 times, in which case your card will be blocked for security reason until midnight UK time
You may have entered incorrect OTP(one-time passcode) in process of verifying MasterCard SecureCode(3D Secure) In which case, you should contact our support line so we can unblock it for you.
The merchant you are trying to pay for might not be currently supported for security reasons. You can find more information in this article here