Trying to reinstall Mac OS10.12.6 Sierra on iMac

If attempting to clear the mac for transfer of ownership, please follow the guidance here:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

To exit the macOS Utilities screen you'll need to shut down and restart the computer.

Do this by clicking the Apple menu () and select your option.

Press and hold the power button until the Mac shuts down.

You can then restart the Mac in Recovery mode again and take steps to reinstall macOS.