The registration needs to be changed to the email that the new owner is using with MAGIX. The old owner needs to contact Magix support for them to release the S/N again, it can then be re-registered to any email address. As the Support document regarding "Error code 24" states: "If the seller was a private person, please contact the seller directly. The seller is obliged to report the sale to us and to delete the registration of the product from his account."
I sold my Vegas 20 serial key to someone else and I'm getting error code -24. Please reset the serial key!
If you transferred your Vegas Pro 20 licence to someone else, of course you no longer have access to that Vegas Pro licence and thus get error code 24. Basically, you no longer own that particular licence because you've sold it to another person. Similarly, if you sell a car to someone else, you no longer have a right to use that vehicle.
Also, please note that this is a user forum - not Vegas Support.