There is information in this link; and additional link(s) to go and get an El Capitan installer
that takes your Mac to the download server; where it can get last OS X 10.11.6 El Capitan.
• How to upgrade to OS X El Capitan (10.11.6) - Apple Support
https://support.apple.com/en-us/HT206886
The above page will suggest you try & get the latest macOS, that may not work on your
older Mac build version; a separate link for 10.11.6 in App store, in blue text, to download.
".. use this App Store link: Get El Capitan .." and this will open up your Mac App store page
~ for just this version Mac OS X 10.11.6 installer. My Mac runs this adequately; & downloaded
the next newest supported installer for later use, I see its in 'my purchases' to consider later.
(Mine can also run the High Sierra 10.13.x system ~ but may be marginal with Mojave 10.14.)
Sometimes those who have issue with an installer not working, once downloaded, can try
and get working after running Safe mode (& restart.) That can fix issues to help it function.
You may need to check the system resources are adequate (unused HDD capacity, etc) &
enough RAM, etc; perhaps get and try Etrecheck for the report, to post in ASC discussion.
There is a pro version you'd not need to buy, the free one works fine; download directly.