Which Magix product are you trying to buy, and from where? And whereabouts in the world are you?
If you are wanting to buy via the magix.com website your card issuing bank has to offer 2-Factor Secure Authentication. This post gives full details.
Is your Mastercard a Credit Card or a Debit Card? It is possible that the latter type is not accepted.
OTOH, it could simply be the situation that there is a temporary problem with either the Magix payment server or your card issuer's server. How many times have you tried to complete your purchase? You could always try again later, as the error message suggests.
If, after trying again later, e.g., tomorrow, you still experience problems, email firstname.lastname@example.org and ask for their help in resolving the problem.