Disk utility won’t mount disk

Try This: Using another Mac, you should be able to create a Bootable Installer of El Capitan.

Once created, boot into Recovery Mode (hold down Command+R at boot) on this computer, erase the disk, and then create a volume of: Extended(Journalized).

From there, try the install off of the bootable installer.

This all comes down to the format of the volume and the format of the installer.

Two Links to Use:

1. Create a bootable installer for macOS --- Create El Capitan Type
2. How to erase a disk for Mac - Create a volume using Extended(Journalized)

If you erased the disk, you can't reinstall when booted normally. So, FWIW, find a way to create one of these USB installers. If you have no way of creating one, then take this into an AASP or your local Apple Store. How to find your nearest Apple Authorized Service Provider (AASP): https://locate.apple.com/country

You can buy a MacOS 10.6 snow Leopard DVD from the Apple online store for about US$20. Delivered by UPS or postal mail. It contains all the needed Utilities and 10.6.3 Installer image.

You said you had tried to Mount using Disk Utility. Where were you running Disk Utility from if not from an Installer DVD?

You can't mount a blank drive. You can only mount a Volume.

Like a Volume from a Drive that has been Erased and had a Volume, such as Macintosh HD, created on it.