If your Mac is not Compatible with Sierra but you Have Not Upgraded to El Capitan
macOS 10.12 (Sierra) requires one of the following Macs:
- iMac (Late 2009 or later)
- MacBook (Late 2009 or later)
- MacBook Pro (Mid 2010 or later)
- MacBook Air (Mid 2010 or later)
- Mac mini (Mid 2010 or later)
- Mac Pro (Mid 2010 or later)
As well as:
- Mac OS X version 10.7.5 or later
- 2GB of RAM
- 9GB of free disk space
However, OS X 10.11 (El Capitan) only requires one of the following Macs:
- iMac (Mid 2007 or later)
- MacBook (Late 2008 or later)
- MacBook Pro (Mid 2007 or later)
- MacBook Air (Late 2008 or later)
- Mac mini (Early 2009 or later)
- Mac Pro (Early 2008 or later)
As well as:
- Mac OS X version 10.6.8 or later
- 2GB of RAM
- 8GB of free disk space
If you have a Mac that is compatible with El Capitan but not Sierra, and have not yet upgraded to El Capitan, it is not too late to do so. You may find that searching the Mac App Store for "El Capitan" will not produce the results that you were looking for, however, you can still download it from the App Store using this link: https://itunes.apple.com/app/os-x-el-capitan/id1147835434?mt=12.
If you had previously downloaded an earlier version, namely OS X Mavericks, Yosemite or El Capitan, you can re-download that version. Make sure you are signed into the Mac App Store with the same Apple ID that you downloaded them with in the past, then open the App Store and go to the "Purchases" tab. If you scroll through your purchases, you should find the OS X versions you've previously downloaded.
