In other words, just because your Mac was compatible with El Capitan (OS X 10.11), Yosemite (10.10), Mavericks (10.9), or Mountain Lion (10.8) may not necessarily mean that you'll be able to upgrade to Sierra.
Some Mac models, such as 20 i Macs, are being left behind with El Capitan as their maximum OS version.
El Capitan requires one of the following Macs with at least 2 GB of RAM, 8 GB of available hard drive space, and at least Mac OS X Snow Leopard (version 10.6.8): If your Mac is currently running Mountain Lion, Mavericks, or Yosemite, then it definitely meets the minimum system requirements for running El Capitan (and it might even be Sierra-capable; refer to the previous section).
If El Capitan is the newest version of OS X that will run on your Mac, but you never downloaded it while it was available in the Mac App Store, you won't be able to find it in the App Store anymore.
For example, the latest versions of Chrome and Firefox won't run on anything older than Mavericks.This didn't just happen once; it has happened again and again.While Apple boasts about the extremely high percentage of i Phone, i Pad, and i Pod touch devices that are rapidly upgraded to each major new version of i OS, such is not necessarily the case with Macs and OS X.Apple has not previously responded to press inquiries about this subject.The good news is that most new Macs sold within the past several years can be upgraded to Sierra.