Factory reset from HighSierra

If you are trying to set up your Mac as it came new, see: What to do before you sell, give away, or trade in your Mac - Apple Support

If you mean something else, please explain.

It should work...

Get an external drive...

https://eshop.macsales.com/shop/firewire/1394/USB/EliteAL/eSATA_FW800_FW400_USB

Once hooked up Clone the Internal drive to the External Drive with Carbon Copy Cloner...

http://www.bombich.com/software/ccc.html

When done Restart & hold Alt key on startup to choose the external drive to boot from.

ok. well there's an easy solution to just wiping the hard drive if that's all you want to do. There's a product I've used for years that is NIST certified to wipe a drive clean. You can download a copy of WIPEDRIVE from White Canyon Software. Go to:

https://whitecanyon.com/home-products

Use the promotion code HOMEFREE to download or pay just shipping cost to get a USB stick loaded with it. The product is free, but pay just a few bucks to ship it.

The copy I've had for years is based on DR-Dos and is self booting. It will find your drive (s) and let you pick which you want to wipe and what algorithm you want to use. The one I used most times is the DOD model. Plan to let it run overnight as it does take quite awhile on even a solid state drive, much less a spinner, and especially if it's multi gig or terabyte.

Hope that helps you out. Perhaps with a totally blank drive you can partition it the way you want and reload your software.

Steve

You can't erase the disk when booted into Recovery Mode? Any error message?

You need to try and explain what happens when you try. We can't see what's going on from here.

Can you boot from an external drive?

When you do that are you pointing your mouse to the drive (top level) or the partition below it (like "MacIntoshHD" or whatever it was called when originally formatted?