No OS on 2008 iMac and no USB boot works

Hello, I am trying to reinstall MacOS 10.11 El Capitan on my 2008 iMac and USB Boot will not show up. I have the application on its own on a USB drive and nothing shows other than recovery when I hold options on the Mac. I have tried resetting PRAM, and I don't know if there is any other way or if I am doing it wrong. USB drive is GUID Partition Map, formatted in the MacOS Extended (Journaled) form.