What you need to do is find your confirmation email for the program you are referring to.
That email will contain a link to the installer for the program you are referring to as well as the serial number for that program.
If you cannot find your confirmation email, then you can log on to your Magix account and grab the installer and the serial number from there (under “My Products”).
It is recommended that you copy and paste the serial number to avoid typos.
It is important that the serial number matches the version of the program you are referring to.
If you are absolutely sure you have the correct serial number, for the program you are referring to, and it’s still not being accepted, then you might need to send an email to infoservice@magix.net to help get this resolved.