Official comment
@YGA > If you correctly entered the license key, it's possible it was already used and then the RB was re-sold to you.
In the meantime, yes, you can use your RB, even without an active license - you can use the 30 day free trial while the license issue gets sorted out. Please file an inquiry ticket here and our rekordbox support team will be able to gather some additional information to assist you in activating your license.