You can enter a product key during installation or run Trend Micro as a free trial. If for some reason (old hardware, perhaps) you must run an antique version of macOS, you might consider ClamXAV (for Mac) or ProtectWorks, which both extend support back to Snow Leopard (10.6), or Intego, which supports Mavericks (10.9) or later. Most Mac users keep their OS up to date, so this shouldn't be a problem. Like Kaspersky, it now supports macOS Mojave (10.14) and later.
