Mac Pro not booting after update
Adventures In Imaging El Capitan Bootable Media
According to this macOS KB Article (https://support.apple.com/en-us/HT201372)

&quot;El Capitan downloads as a disk image. 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. You will create the bootable
installer from this app, not from the disk image or .pkg installer.&quot;

I just attempted creating a bootable  on Catalina and when running the PKG it says it will not install on this Mac as it is not compatible.
So I attempted to work around that problem. I had to use CharlesSoft's Pacifist utility which allows the extraction of any type of Mac Package file format. Since I am a MacSysAdmin I have this utility already and it comes in handy all the time.  Unfortunately, it's not open source and it's not free. But for $20 shareware it does what it says it does and it does it very well.
I was able to extract the PKG from the DMG:
InstallMacOSX Folder
  └──InstallMacOSX Folder
       └──Install OS X El Capitan

Then I moved &quot;Install OS X El Capitan&quot; to &quot;/Applications/Install OS X El Capitan&quot; and then I ran the El Capitan &quot;createinstallmedia&quot; command.
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app


Guess What? That Does Not Work!
I even tried using DiskMakerX ver 6 and ver 9 and it didn't work either. Stated my El Capitan was an incomplete download which it is not an incomplete download.
So back to the drawing board. I try it on Mojave instead. My Mac is dual boot Mojave / Catalina. So in Mojave, it still doesn't work.
Copy it over to the High Sierra Mac.  Finally, some success. The DMG PKG file installed in /Applications/Install OS X El Capitan and I was able to burn a flash drive.
It Only Works on macOS High Sierra or older
So your options are to find someone with a High Sierra Mac or maybe you could install VirtualBox (free), Parallels, VMware Fusion and install High Sierra as a virtual machine then use that to burn an installer. FYI, it can be tricky to access a physical USB in a virtual machine.
Once you get it working, hang onto that working boot USB drive since it's such a terrific pain to build one!  The way the El Capitan createinstallmedia works, varies considerably from the newer macOS versions. It doesn't have the normal InstallESD.dmg, etc.