Essentially, the code means you are entering a correct serial number but for the wrong product. Magix Serial Numbers are always product and edition specific, so the code you use must be the correct one for your installation of "Magix PC Check 2021" and not any other edition of the software.
Also, can I recommend removing your personal email address from your User profile? These are public forums so accessible to anyone, including BOTs, who may be delighted to harvest another email address to which they can send potential spam!