Where, exactly, did you buy this software? If it was from a private seller, so used or second-hand, you must contact the previous owner and ask that he inform Magix that he no longer owns or uses the product and to request the serial key be removed from his account. This is because serial key is always permanently linked to the email address the original purchaser used and only the owner of that address can make any changes to those details.
If you bought the software brand new, i.e., you are the first purchaser you need to check that the email you used when buying the software is exactly the same as the one you are using to register it.
Looking at the web page for the store where you bought the software it would appear you have bought a boxed version. Is that correct?
In this situation it is the responsibility of the seller (Elgiganten) to ensure that your software works as it is supposed to, including you being given a serial key which has never been used before. I wonder whether you have been sold a copy which someone else has previously bought, apparently installed and registered the software and then returned the box to the seller, presumably in order to receive a full refund?
Because Magix will have no record of your purchase only Elgiganten can help you to resolve this. I think you need to go to the store, explain what the problem is and ask for a brand new box of the software. With that box you should receive a new serial key which you can then register.