The macOS 10.11 El Capitan installer is a bit more complicated. First the installer downloads outside of the App Store in a .dmg file. After mounting the downloaded .dmg file you are presented with an "installer" app in a .pkg file which only extracts the real installer to the Applications folder. After this finishes you need to launch the real installer from the Applications folder. It should be called something like "Install macOS El Capitan".
This Apple article includes links to download macOS along with instructions on how to extract & use the macOS El Capitan installer as well as option instructions for creating a bootable macOS USB installer:
https://support.apple.com/en-us/HT201372
Make sure to have a good backup before attempting to upgrade macOS just in case something goes wrong.