error "unable to unmount disk" to erase an I Mac

It appears you have just a single hard drive installed in your Mac. Since you have a hard drive I recommend enabling Filevault and let the Filevault encryption process complete. The encryption process can take a day or more depending on the health of the drive.

I then recommend creating a bootable macOS 10.11 El Capitan USB installer using these instructions (read them carefully as there are multiple steps to extract the installer to the Applications folder):

https://support.apple.com/en-us/HT201372

Using the bootable macOS USB installer is best since you will be erasing the drive and will want to reinstall macOS for the new owner in order to show the laptop is functional. It should also be possible to boot into Internet Recovery Mode using Command + Option + R, but Apple's information on this is a bit confusing for an older system only running macOS 10.10 and many people have posted issues when using local recovery mode to perform a clean install like this.

Here is an Apple article on what you should do to prepare a Mac for sale or disposal:

https://support.apple.com/en-us/HT201065

Unfortunately Apple's article neglects to show the best way to securely erase a hard drive (Apple's instructions are fine if you are using an Apple SSD due to how SSDs work differently than a hard drive), but if you follow my advice to first enable Filevault and letting the encryption process finish, then the rest of Apple's instructions for erasing and reinstalling macOS will be just fine since your data will have been destroyed when the Filevault encryption key is destroyed when using Disk Utility to erase the drive.

Edit: I should add that you must be booted from external media in order to erase the drive. You cannot erase the drive that you have booted from. This is why it is best to use a bootable macOS USB installer (or Internet Recovery Mode).

Follow this article, instead of erasing the disks reinstall the OS: What to do before you sell, give away, or trade in your Mac - Apple Support

Launch the Terminal app and run the following command and post the results here so we can see exactly how your system is configured.

diskutil  list  internal

A single pass of zeroes is more than sufficient to securely erase your personal data. Unfortunately macOS doesn't provide an easy way to verify the hard drive is filled with zeroes since most of the low level utilities are not contained within the macOS installer (or recovery mode).

You may as well just install Snow Leopard. I don't think you have access to any type of recovery mode unless the Mac had macOS 10.12.6+ installed at some point, but Apple's documentation is a bit unclear on this. When the laptop reboots into the new OS and begins to run Setup Assistant select Shutdown so that the new owner will be greeted with Setup Assistant. I haven't installed Snow Leopard in a really long time so I don't recall whether you may need to select a language or other options in Setup Assistant before the Apple menu appears so you can select "Shutdown".

What version of macOS are you booted into when attempting to run the command?

If you disconnect all external drives, then you can run the command without the "internal" option if you are booting to an older version of macOS (I think any version of macOS from 10.10+ allows the "internal" option to be used). You need to take a screenshot or picture of the Terminal window showing the command you used and the results since we may be able to spot any mistakes plus we need the exact details of the results to help you.

The advantage of using a USB installer is that you can install the later version of macOS for the new owner and the USB installer gives you more options in case you find the Snow Leopard DVD fails midway. Creating a macOS USB installer is tricky if you erase this older Mac and don't have another compatible Mac which can create a USB installer. Depending on the exact model of your Mac it may be capable of running a newer version of macOS such as 10.13, but we would need to know the exact model. Most 2010 - 2016 Macs can run macOS 10.13 (sometimes even a late-2009 model).

You can definitely use a Snow Leopard DVD if it is the correct one and your optical drive works. The older version of Disk Utility has an option to securely erase a hard drive by writing zeroes to the drive if you are unable to enable Filevault. I think it is under an "Advanced" option (it has been a long time I used an older version of macOS with that option).

Try typing Command + Q which should present you with a quit option for Setup Assistant (I believe this may have been necessary on the older versions of macOS) if you cannot select "Quit" from the Setup Assistant menu or "Shutdown" from the Apple menu.

You can Option Boot the Mac to let the Mac sit at the Apple boot picker menu where you should be able to eject the DVD or you can hold the left mouse button down as you power on the Mac.

Edit: Don't register the product. It should be possible to skip the registration portion (maybe with Command + R?). Don't create any user accounts as that should be left for the new owner. If none of these suggestions help to shutdown macOS cleanly, then just force the Mac off by holding down the power button.

Does the DVD even attempt to eject itself when you try these methods? If it tries to eject, but cannot, then there is a blockage with the CD slot. Sometimes the CD slot can become compressed or the felt around the CD slot can become stiff both of which can prevent a DVD from ejecting. Sometimes the CD slot & felt can be carefully adjusted to allow the DVD to eject.

Unfortunately if you cannot get the DVD to eject by holding down the left mouse button (allow the system to boot into Setup Assistant again) or by sitting at the Option Boot screen (boot picker menu) and pressing the "Eject" button on the Apple keyboard, then it may not be possible to removed the DVD without taking the Mac & optical drive apart to physically remove the DVD.