Factory Reset iMac OS X 10.6.8

Buy Mac OS X Retail v10.6.3 from eBay.

Or Mac OS X Server v10.6.3 rev 1 Snow Leopard Unlimited Clients MC588Z/A. It might be more expensive but it can be virtualized under later macOS like Sequoia via VMware Fusion etc (only in Intel Macs, though).

I'm still working one MacMini 2010 on 10.6.8 (making money on it with CS6 and GoLive that my other Macs won't run)

I recall being in the same position

and installing 10.6x from an install disk was tricky

it has to be the same OSX retail version or greater than the one your Mac shipped with (if it shipped with Snow Leopard)

or specific OEM installer paired to your exact model (which will also have the exact version your machine requires to authorize the installer)

once you get 10.6x installed -- then -- you run the 10.6.8 update (I would look for the 10.6.8 combo updater)

THEN YOU RUN TWO CLONES FOR BACKUP (when your working system goes south, or you have to replace the machine, you just erase and run the clone or swap out one of the backup HDD)

10.6.8 should boot any Mac that will run SNOW LEOPARD

10.6.0 retail should install on any Mac that shipped with an earlier OS (that is capable of running 10.6x)

+++++

I recall I had to install 10.6.0 retail (see pic) to an older machine, update it to .8, and clone it over to the 2010 Mac Mini (or the MacMini shipped with an older OS and it took the .0 installer) -- but I remember finding a legit copy of the installer was a real pain...

March 15, 2010 to July 21, 2011 Macs, you need the system specific installer disc or flash drive that came with the Mac labeled for the same model, and it will be 10.6.3 or later. These models once they have a firmware update installed will also support recovery: Computers that can be upgraded to use OS X Internet Recovery - Apple Support

August 28, 2009 to March 14, 2010 Macs will support the 10.6.3 retail which looks like the above only the black letters say 10.6.3.

January 7, 2006 to August 27, 2009 Macs all support the disc shown above.

The recovery system didn't become available until 10.7. Reinstalling the OS requires a compatible physical Mac OS X installation disk.

(257403)

If you don't have the one that was used to upgrade it to Snow Leopard, you'd need to buy another. Ones intended for a different Mac model can't be used for the installation.

(257405)

still if a Mac shipped with 10.6.4 or greater pre installed, it won't boot (or install) from a 10.6.3 retail snow leopard install DVD (in my experience)

perhaps the OP post their system hardware overview and model year it would be to find what OSX version it shipped with (excuse if I missed that info)... erase reinstall start from scratch