USB Bootable El Capitan.

Did you note these specific guidelines below (numbers 1-3) from How to download and install macOS - Apple Support?

1. Use these links to download a macOS disk image (.dmg) file.
2. Double-click the .dmg file to open it and see the package (.pkg) file within.
3. Double-click the .pkg file, then follow the onscreen instructions to install the macOS installer into your Applications folder...

(The above are found mid-page in that document, under the heading "Use a web browser" and relate to downloading the older versions of macOS from Lion to Sierra and including El Capitan.)

The primary prerequisite is that you need to execute that command on a Mac that is actually capable of running OS X "El Capitan". That is what is meant by,

"The Mac must also be compatible with the macOS that you're downloading."

This limits the number of Mac models that can be used to a specific range of models, which you can research on your own (but please write back if you are unsure).

Then, you have to mount the .dmg (disk image) to reveal the "Install OS X El Capitan" app before you can execute that command. Merely renaming the .dmg file won't help.

Then, it should work.