Version 16.0.26.0 includes advanced features that require a genuine license to function correctly:

Check reputable retailers (like Amazon or Newegg) which often sell physical or digital licenses at a lower price than the MSRP. Features of ESET Internet Security 16.0.26.0

Many keys shared on public forums are bundled with "activator" tools that contain Trojans or spyware designed to steal your personal data.