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).