Bootable USB drive with El Capitan will not boot.

I have a 2017 21.5" iMac that I want to create another partition and install El Capitan on to use some older software that I do still use, but rather infrequently.

I followed the directions in "How to create a bootable installer for macOS" and it created the bootable USB drive fine, with no errors, but when I restart the iMac and hold the option key down after I hear the startup chime in can navigate over to the El Capitan USB drive and when I click on it I get a circle with a line thru it.

I have created the USB drive 3 times using different downloads of the "Install OS X El Capitan" app and I get the same thing each time I try.

What could be the issue?

Thanks in advance;

Steve