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?
Here are the instructions for creating a bootable macOS USB installer:
Install El Capitan From USB Boot Disk Onto Erased Hard Drive. If playback doesn't begin shortly, try restarting your device. Videos you watch may be added to the TV's watch history and influence. Jun 22, 2020 I've forwarded the early 2008 machine drives (unaware than getting the installer is going to be so challenging). I am unable to open the El Capitan package to get the installer on my 2019 machine. I just need a bootable version of El Capitan, or to install on an external hard drive so I can reboot my early 2008 machine. Install Diskmaker X to the Applications folder. Download the El Capitan installer from the App Store and then select El Capitan in Diskmaker X. We'll use a flash drive but you can.
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.
Download From Usb
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.
How To Install El Capitan On External Drive
Aug 21, 2021 12:52 PM