How to reinstall macOS on my iMac

I have a 2017 27" iMac that I have replaced. I would like to return it to Apple for recycling. Using Recovery mode, I think I have erased the hard drive. When I attempt to reinstall the macOS, I can not seem to do it. Do I need to reinstall the OS before taking it to Apple? How can I be sure that the hard drive is erased?

[Re-Titled by Moderator]