How are you trying to download El Capitan, not from the Apple Recovery Servers I hope.
Does your mac have a working OS, if so there is no need to use the Recovery Servers, the
copy of El Capitan they offer has an expired certificate that Apple seem to be reluctant to do anything about.
If you do have an operating system on your mac and it is functioning then you can download
El Capitan using this method.
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
