All that should be needed is the following:
1: A machine running Server.app or any machine running System Image Utility
2: Download the El Capitan installer from the App store and have it present in the Applications folder
You have four choices. NetInstall, NetRestore, NetBoot, and Package only. NetInstall is the easiest because you only need the El Capitan installer. Yes, you can add additional items to the payload, but at the basic, all that is needed is the Installer present in the Applications folder of the machine running System Image Utility.
NetRestore images assume you have created a master image that is than captured to the NetInstall service. NetBoot allows the capture of a prepared drive for booting and package only will deliver installation pkgs to the target machine when booted from the network.
I am guessing you forgot to download the El Cap installer.
Reid
Apple Consultants Network
Author "El Capitan Server – Foundation Services" :: Exclusively available in Apple's iBooks Store
Author Mavericks Server and Yosemite Server series, also available in Apple's iBooks Store