You could likely get the upgrade to El Capitan 10.11.6 from the App Store
if the computer has adequate resources to run that late a system...
In case you have OS X Snow Leopard or Lion, but want to upgrade to macOS High Sierra, follow the steps below: To download Mac OS X El Capitan from the App Store, follow the link: Download OS X El Capitan. On the El Capitan, click the Download button. Next, a file Install OS X El Capitan will download into the Applications folder. The El Capitan installer is downloaded to the /Applications folder, with the file name Install OS X El Capitan. If you have already installed El Capitan and want to now create a bootable installer, re-download the installer from Apple.
'If your Mac doesn't meet the Sierra requirements, you might be able to upgrade to El Capitan,
the previous version of the Mac operating system.... If your Mac doesn't support Sierra, or you're
using Snow Leopard & would like to upgrade to Sierra, you can get El Capitan from the App Store.'
-- blue-linked text goes to Apple support content.
...The link to get El Capitan from App Store seems to go there, see if your Mac can use it.
Good luck & happy trails! 🙂
Mac Os X El Capitan Not Available In App Store
Nov 11, 2016 7:32 AM