I can't make bootable USB on 2008 iMac 2008 running El Capitan
OS X El Capitan has a recovery partition which is used to reinstall OS X on your Mac should you need it. Instead of using the .pkg, You can try copying the entire contents of your recovery partition to your USB. After formatting the USB as macOS Extended, go to OS X Recovery by pressing Command + R. Then, unmount all volumes except the Recovery partition and USB and go to Terminal. Then type
cp / /Volumes/USBNAME.
If you are trying to make a ready installation of OS X El Capitan, then you can use your recovery partition to reinstall OS X to a USB drive. Simply boot into OS X Recovery and select the reinstall OS X option. Select your USB drive and connect to WiFi if prompted. If you would like to install macOS Catalina on the USB, then Apple says you must upgrade your computer to macOS High Sierra to avoid issues with the firmware and in order to have access to the Install macOS Catalina application.