Reinstall OS X for older MacBook Pro
You can download OS X El Capitan 10.11.6 (6.21 GB) via this link, which will open first in the Browser, then in the App Store.app.


  OS X El Capitan
  
  This version of OS X El Capitan is for users running OS X Snow Leopard
  who would like to upgrade to macOS Sierra. If you are running Snow
  Leopard and would like to upgrade to Sierra, you need to install El
  Capitan first. This version of El Capitan can also be installed on Mac
  computers that are not comparable with Sierra.