Do not try to install the version of El Capitan downloaded from the App Store Purchased page
it probably has outdated certificates.
There is no need for you to install all the OS X's to get to El Capitan you can install directly from
Mac OS X Snow Leopard 10.6.8. After you have installed Snow Leopard you need to update it to
version 10.6.8 from here,
https://support.apple.com/kb/DL1399?viewlocale=en_US&locale=en_US
And if the installation fails you should not have to erase the mac and reinstall Snow Leopard.
If you do have an operating system on your mac and it is functioning then you can download
El Capitan using this method.
Open your Safari browser and click on the link below,
How to get old versions of macOS – Apple Support
Go to Download OS and click on OS X El Capitan 10.11
this downloads InstallMacOSX.dmg to your Downloads folder.
The next section can only be done on a mac that is capable of running El Capitan.
This includes macs that have the potential to run El Capitan but have been upgraded to a newer OS.
A mac that came preinstalled with an OS later than El Capitan will refuse to do the next bit.
When downloaded open to InstallMacOSX.pkg, double-click on
that and an installation window will open, this does not install El Capitan
but converts the InstallMacOSX.pkg to the Install OS X El Capitan.app which
you will find in your Applications folder, it should be 6.2GBs in size.
(If the installation window asks which disk you want to install to, you must pick
the disk that you are booted to at the time. Not any internal or external disk that
you want to eventually install El Capitan on, that is for later.)
To start the installation of El Capitan double click on the Install OS X El Capitan.app.
The copy of the install app self deletes after installing El Capitan, so make sure you keep a copy of the
InstallMacOSX.dmg if you need it in future, or you could just make a copy of the Install OS X El Capitan.app
prior to installing and moving it to an external drive for safe keeping.
It is also possible to create a bootable USB installer disk using the Install OS X El Capitan.app in the Applications
folder and the createinstallmedia command in the Terminal app.
Read the instructions here,
How to create a bootable installer for macOS – Apple Support
