If the hardware is capable of a system software upgrade, or has minimum
specifications to run El Capitan 10.11.6 (with adequate RAM memory or
other hardware upgrade to the Mac) you may be be able to use the linked
download to get El Capitan from the following Support page:
• Upgrade to OS X El Capitan - Apple Support
The App Store link goes direct to view and download El Capitan 10.11.
https://itunes.apple.com/app/os-x-el-capitan/id1147835434?mt=12
Note the generally low stated minimal specification suggested in order to
run the Mac OS X 10.11.6 are just that. Minimal and for that OS X only.
When running and under a load with other Applications, the numbers may
indicate an upgrade to more RAM memory capacity, as well as in Storage
hard drive GB capacities, is forthcoming. These numbers do not interchange.
Because you've not stated the actual build model identifiers, there may be
more that could be done to check before attempting to install an upgrade.
Good luck & happy computing! 🙂