The edition of Magic Desktop you have doesn't match what the key is meant to activate.
There are two main editions:
- one for families, which is the standard experience (https://www.magicdesktop.com/go/download);
- another for organizations, which is the Kiosk release, designed for public libraries and other venues with patrons (https://www.magicdesktop.com/kiosk).
Please review your license information on https://my.magicdesktop.com/ or on your receipt e-mail that contained the key. If it turns out the edition you have is incorrect, install the correct one over your current setup and you should be able to activate it once it's done.