If you don't have a serial number on file, and you don't remember buying a version since you started using the software in 2017, chances are you are an early user of the free Music Maker version (25.0.x) that introduced the new "freemium" concept. With that version, MAGIX started pursuing a new concept with the Music Maker product line of offering a completely free base product, which can then be expanded with audio content/loops and virtual software instruments via the built-in store system. After another update we put out for that version (to version 25.1.x), we added further functionality and content. You were then able to also purchase entire editions of Music Maker through the built-in store, which are essentially another way to get credit for picking audio/loop content, software instruments and program features. There is now basically just one single version of Music Maker, which makes it easier to maintain and ensure that every customer is using the most recent version. This version is then customised by the user according to their individual needs, by just adding the particular audio content, instruments and program features they want. The function to enter a serial number is just needed to unlock the credits for the editions mentioned above (if purchased outside of Music Maker), or can also be used to unlock features/instruments from older Music Maker ans Samplitude Music Studio generations that some users may own. You don't necessarily need to enter anything here, if that doesn't apply to you. You would still be able to keep using the free version, and obtain add-on content or features through the built-in store system, if required.