Comentário oficial
@Jordan > You can install the license on up to 2 computers. Had you ever installed it on another computer? Or did you possibly update the operating system in that time?
You can figure out which computers have licences installed by going here:
https://rekordbox.com/en/account/profile/
Login, then click on the Licenses tab on the left.
If you see the matching serial number with two computers you'll need to deactivate one of those installations first.
If you no longer have access to either of those computers, you'll need to contact our rekordbox support team to have the license deactivated for you.