Below is an overview of how the current licensing system works and why you don't need a traditional serial number anymore. 1. The Death of Serial Numbers
Virtual DJ 5.7 is a legacy version from over 15 years ago, and modern versions of the software no longer use serial numbers for activation. Today, VirtualDJ is a "live" service tied directly to your user account.
Old versions like v5.7 were deprecated in 2014. Most modern licenses (Home or Pro) are designed for VirtualDJ 8 and newer. 2. Modern Licensing Options