I'm a bit confused by your post on what you are trying to do here. Are you trying to boot into Recovery Mode to install macOS to an external USB drive or are you trying to boot from a macOS USB installer to install macOS to the internal drive?

1. Apr 27, 2021 Download: OS X El Capitan This downloads as a disk image named InstallMacOSX.dmg. On a Mac that is compatible with El Capitan, open the disk image and run the installer within, named InstallMacOSX.pkg. It installs an app named Install OS X El Capitan into your Applications folder.
2. How to create a bootable MacOS USB.
3. Install & Download DiskMaker X For OS X 10.11 so that you can easily make a USB Bootable disk. We also have screenshots.

Here are the instructions for creating a bootable macOS USB installer:

As usual, Apple no longer supply a bootable USB Flash Drive with latest OS X releases - including the new OS X 10.11 El Capitan. Using this tutorial you will. Download El Capitan OS X is also significant since it’s the last version of Apple Mac OS X releases as its successor Sierra was released in the year 2016 under the brand macOS.

Make sure to carefully read the section regarding macOS 10.11 as there are extra steps required to extract the real installer from the downloaded .dmg archive to the Applications folder. After running the .pkg file from the mounted .dmg file you can then proceed to use the command line to create the bootable macOS USB installer itself.

Macs are very picky about the drives used for booting a Mac plus the quality of many USB sticks is extremely poor so try using another USB stick especially a different brand.

When erasing the drive make sure to select the whole physical drive within Disk Utility and erase the physical drive as GUID partition and MacOS Extended (Journaled). The physical drive should be the top most item which is usually identified by the make & model of the drive itself.

Apple Support Make El Capitan Bootable Usb

Aug 21, 2021 12:52 PM

Download El Capitan To Usb

This is Ultimate tutorial for installing Mac OSX up to EL Capitan 10.11.
You don't need working mac installation or AHCI capable motherboard, it can be done on any PC.
Tools that you need:
OSF Mount - http://www.osforensics.com/tools/mount-disk-images.html - This is most important tool, it can mount efi partition on image and edit it.
image link: https://goo.gl/nMcpOG
win 32 disk imager - http://win32-disk-imager.en.uptodown.com/ - this will be used to write image to USB, you can also use other software, there are many of them, i use this one.
In case you did not listen to me and you want to do this harder way:
http://www.hackintosh.zone/file/828-bootdiskutility-v212015/ - utility that can install clover from windows.
https://sourceforge.net/projects/cloverefiboot/files/Installer/ - Clover installer package
The most important note!!! ---- You need Sata kext in 10.11 folder, that will make your HDD to show up in disk manager, without it you will get still waiting for root device and your HDD wont show up in disk utility and you wont be able to install EL Capitan on your HDD.
My Specs :
Processor - e7200 core2duo
Ram - 6gb - different modules with different speed and latency
HDD - 320+720gb - sata2
USB - 16gb Kingstone
GPU - HD 7850 2gb dual bios.
Motherboard - P5N-D - https://www.asus.com/Motherboards/P5ND/