It is very difficult for fellow-users to advise on registration issues because these are often unique to the host computer. If you both have a Magix Account I would suggest you raise a Support Ticket and ask for help there.
One possible reason for @Richard-Peach; there is an activation limit on the number of installations that a user can have simultaneously. If that is the problem, you should be able to log into your Magix Account, go to My Products, find the entry for XDP and check the number of activations listed. Remove the activation(s) you no longer need, then the new install should be able to be registered.
@jerry-anderson, a couple of possible reasons: have you just bought the software? From where? Have you tried copying and pasting the serial number into the software?