First, we need you to clarify some things: Magix is the company who manufacture the software Sound Forge.
Please tell us exactly which Magix product you have bought and from where. We can assume it is Sound Forge but the actual place of purchase is very important.
Also, the message of an invalid serial number would suggest you are trying to use one that is applicable to something like Sound Forge Audio Studio but have actually installed Sound Forge Pro, or vice versa.
@Paul-Godwin Please check the same e-mail for additional serial numbers. Every software has its own serial. By the way, is Sound Forge Audio Studio 17 even included in Music Maker? I don't know.