Trouble doing clean install of El Capitan

Will defer to @Eau Rouge but perhaps the solution is to create a USB Installer of El Capitan. Once done - will have to do the Terminal Time / Date 2017 thing to get it installed as per @Eau Rouge instructions.

Did you follow this procedure,

The certificates for several of Apples OS's expired in October 2019, Apple have been very poor at getting these updated on 

their Recovery Servers with valid certificates. 

This may be the problem you are experiencing. 

Try this workaround, we are going to set the time and date on your mac to a date previous to the expiry date of the certificates.

Connect your mac to your router via cable, not WiFi

Boot to your Recovery HD, click on Utilities in the menubar select Terminal.

Make sure WiFi is switched off, it can reset the date back to today.

Enter a new date, for example or just copy and paste

sudo date -u 011421002017

press Return

enter your password

press Return

If Terminal returns an error saying sudo : command not found, then try again without sudo.

Now try downloading the OS.

If this works then when the OS is installed and booted up you can Open System Preferences> Date & Time

and reset the time back to today.

One would need access to another Apple Computer compatible / able to run with El Capitan to download the DMG, install the App on that computer and then create the USB Installer.

Very Welcome and pleasure to find a solution to the Issue.

Thank you for marking it as such.

Agree with hoops required to jump through to achieve but at sometime - good learning curve for us all.