reinstall el Capitan and mojave

It would therefore appear the user ( you ) are using a Computer that Does Not Qualify to Run Mojave or El Capitan.

OS X El Capitan - Technical Specifications and if the computer is Not Specifically Listed - that computer can not be used to Download and / or Create a Bootable of El. Captain

macOS Mojave - Technical Specifications and if the computer is Not Specifically Listed - that computer can not be used to Download and / or Create a Bootable of Mojave

To perform this action will require booting from a Bootable Installer. 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Example : Bootable Installer of XXX ? would have to be done on a computer that Qualifies to run XXX.

Therefore - if the computer being used to create a Bootable Installer for either El Capitan and / or Mojave is too new - this is the wrong computer to perform these actions .

It does not matter that - making a Bootable Installer - as per the " Apple Licence of Use " prohibits this computer from downloading or installing or downgrading or making a Bootable Installer for another computer.

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence

To perform this action will require booting from a Bootable Installer. 

My various machines are all perfectly apt to use both El Capitan and Mojave.

Also following the link you suggest it leads to a page where I can upgrade El capitan not FRESH and CLEAN install it.

As for MOJAVE as I did some updates, the page tells me I cannot update MOJAVE as my current verision is too new.

Youj are always very kind Mr. Pjillips, but I am in a terrible situation as I need to send a screenplay else I will loose the job and I don't know how to do it.

Because of the COVID19 I cannot go t a friend with MAC bringing my portable HD so I am rather desperate.

I recall seeing years ago some software which was meant for People on Windows, and they had a link to install both some software which would automatically create the ssd or hd with the older version, and also allowed to download an ELCAPITAN full verion.

DO you know anything about it?

Thanks for your time and kind regards

When the El Capitan is downloaded it would come as a DMG file. That file needs to opened. Once opened and PKG file will be inside. If that is present double click the PKG and that will place a new application inside Applications folder called Install El Capitan.

Then and only then run the Terminal Command without Quotation Marks. The Untitled in below command is the Actual Name of the External Drive that needs to formatted as HFS Journaled and GUID Partition Map.

" sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app --nointeraction "

Though, I doubt any of this will succeed for the reasons already presented earlier.