I am Unable to install OS X EL Captain from OS X Snow Leopard

Have you checked that your version of Snow Leopard is fully up to date, check in the App Store Updates.

Open Disk Utility and verify the disk for any errors.

Ok if you can't be bothered re-downloading it, go to the other mac you had no trouble with.

Do you still have a copy of Install OS X El Capitan.app then put it in the Applications folder.

(if you don't have Install OS X El Capitan.app then make it again using the InstallMacOSX.dmg)

Presuming this other mac has got El Capitan installed you can create a bootable USB installer

using the command in this article,

How to create a bootable installer for macOS – Apple Support

You will also need a USB stick 8GBs or over, reformatted in Disk Utility

as per the screenshot.

After the USB has been reformatted then you can open Terminal and copy and paste this command into the Terminal window,

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

press Return,

enter your password,

press Return,

enter Y

press Return,

Terminal will now create the bootable installer, be patient it takes about 30 minutes.

Having created the USB installer then remove it from your mac and plug it into

the first mac.

Restart your mac while pressing and holding down the option/ alt key.

After a couple of minutes you should see the Startup Manager.

Select the USB (Install OS X El Capitan), press Return.

The mac will now boot to the USB, a Utilities panel will open,

select Install OS X, press Continue.

The Installer will open follow the prompts.

Looks like you have downloaded the El Capitan InstallMacOSX.dmg

is it 6.2GBs. You may need to download it again, this time make sure

your mac is connected to your router via cable.

If the download has been compromised you will just have to re-download it.

follow my instructions.