Yeah I think you will have to reinstall Snow Leopard.
Once you have installed it then update it to version 10.6.8.
Check the internal drives health using this app https://binaryfruit.com/drivedx
there is a free trial period.
El Capitan may not be installing because of a failing drive,
Disk Utility only checks the disk directory so is not really checking the disk.
After you have installed Snow Leopard and got it up to 10.6.8
you can think about installing El Capitan.
This time download El Capitan from here,
How to get old versions of macOS – Apple Support
Go to Download OS and click on OS X El Capitan
this downloads InstallMacOSX.dmg to your Downloads folder.
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
