Use the 'createinstallmedia' command in Terminal. You will create the bootable installer from this app, not from the disk image or.pkg installer. It installs an app named Install OS X El Capitan into your Applications folder. On a Mac that is compatible with El Capitan, open the disk image and run the installer within, named InstallMacOSX.pkg.Turn on your Mac and continue to hold the power button until you see the startup options window, which shows your bootable volumes and a gear icon labled Options. Plug the bootable installer into a Mac that is connected to the internet and compatible with the version of macOS you're installing.You will then be prompted for your user password (or, if you have a touchbar, to touch the fingerprint sensor). Getting ready to create the bootable USB drive.
