Running High Sierra, need to reinstall El Capitan on different iMac

The one you downloaded from the Purchased section in the App Store may not be the revised version, and may result in error.

I know for certain that the download offered through the Upgrade to El Capitan link I given you twice now is the new version with the updated certificates. I have explained how this works very clearly so I do not understand your confusion.

If your mac is running Snow Leopard make sure it is version 10.6.8, if it is not download this,

https://support.apple.com/kb/DL1399?viewlocale=en_US&locale=en_US

then check the App Store for any other updates available to you.

Why do you want to do a clean install, just install it over Snow Leopard.

With the recently updated version of El Capitan with new valid certificates released in October 2019 Apple

changed the way you downloaded El Capitan. From How to upgrade to OS X El Capitan – Apple Support

go to Section 4 and click on Download Mac OS X El Capitan. This will download InstallMacOSX.dmg to your

Downloads folder.

Opening this will open to the InstallMacOSX.pkg.

Double-click on that and an installation window will open, this will convert InstallMacOSX.pkg to the

Install OS X El Capitan.app which will be in your Application folder.

Use the Install OS X El Capitan.app to create a new bootable USB installer,

How to create a bootable installer for macOS – Apple Support

If the Mac that you are downloading this to is a Mac that came with a newer OS than El Capitan preinstalled you

will not be able to do the conversion to the Install OS X El Capitan.app. The procedure needs to be done on

a Mac that would have at sometime in its life been able to install El Capitan.

Did you partition and format the external SSD? Are you booting your computer from the El Capitan Installer? When the Installer starts up it asks a couple of questions first, then shows a HDD icon. Below the icon is a link labeled, "Show other disks." Click on it to find your external SSD.

Drive Preparation - El Capitan or Later

1. Open Disk Utility in the Utilities' folder.
2. After Disk Utility loads select the drive (out-dented entry with the mfgr.'s ID and size) from the side list. 
3. Click on the Erase button in the Disk Utility toolbar. A panel should drop down.
4. In the drop down panel set the partition scheme to GUID. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
5. Click on the Apply button and click on the Done button when it is activated.
6. Quit Disk Utility.

Well it does help if you follow all the steps that have been suggested.

Ok if you are replacing the hard drive you may as well go for the clean install.

Boot to the USB, select Disk Utility, highlights the Disk click on Erase,

give the disk a name, Format: Mac OS X Extended (Journaled), Scheme: GUID Partition Map.

press Erase. When that completes quit Disk Utility.

Now select Install Mac OS X, follow the guides, select your Disk, should take about 15 minutes.