Please make a screenshot of what you are doing and the error message and add it to your comment with the upload button. With your short description I am not able to imagine the what you might be doing wrong or what Music Maker is doing wrong.
PLEASE cover up any mail or serial number in your screenshot.
If you sign in with your mail and password on Magix.com does it work? If you then look under ''My Products' are both Music Maker Premium versions listed? You can then click on these listed versions. Are these versions already registered to a computer? Then maybe try to deactivate them and try again to log in on your computer.
If there are no versions listed you can register both your premium versions in your account via 'Product Registration'. Maybe this will work.
That's very odd. Maybe it's a technical problem on Magix side. You can contact infoservice@magix.net They should help you with registration problems. Send them all your data and maybe they can do something. For now I'm out of ideas.