Can't "find" HD for install

I was trying to erase the Fusion drive on my mac mini ready for a fresh install of El Capitan.

I had booted into the USB installer for El Capitan and ran Disk Utility to erase the Macintosh HD. It went through the process and the scroll bar ran across the screen before it finished and came up with an error warning that it failed to erase the drive.

The drive list refreshed on the left hand side and instead of the "Master drive" with "Macintosh HD" drive as the partition underneath, it just has Macintosh HD.

If I then go to try and install the OS the OS can't "find" the drive to install to but it can see the installer drive.

If I go back in to disk utility I can get info on the disk and Verify and Repair the disk and am told the disk is fine. But everything else is greyed out.

I found some directions about wiping the fusion drive by going to terminal, looking for the disk volumes "diskutil cs list", then "diskutil cs delete ID ...", to then do a repair in Disk Utility which should realign the drives. The only problem is once I've found the drive ID then try to delete it, Terminal comes up with this in the last line "Ownership of the affected disks is required". But I don't seem to be able to do anything about it.

I can't get back into OSX as it's been deleted to do anything in OSX.

Is there anything else I can or need to do? Or do I just need to take it to a service repair person.

Also. If I go in to Terminal I am met by "-bash-3.2#". I read something that I should have a "$" and that I need to exit from that prompt by typing "exit". Once I do that I'm asked to "Please Confirm" but I can't enter any more text as my keyboard is unresponsive.

Any help appreciated.