Which version of MM do you have and where did you buy it?
The fact that the Serial Key is already in use suggests, either, that you bought the product from a previous user, who has NOT informed Magix of the sale and so the Key is still registered to his account; or, that you have previously registered the purchase with a different email address to the one you are now using.
If you can tell us which, if any, of the above two scenarios apply to you we can advise you how to rectify the problem!
...or that you have already activated the serial number in Music Maker. You only need to do it once. If you do it again, it will give you the message that you got. The Serial Number box is there in case you want to activate a new one, not an old one.