Cant see erase from disk utility

You don’t want to erase a OS disk while you are running it. This is the way. What to do before you sell, give away, trade in, or recycle your Mac - Apple Support (CA)

6. Erase your Mac

1. Shut down your Mac.
2. Start up from macOS Recovery, then use Disk Utility to erase your Mac. For details, follow the steps to use Disk Utility to erase your Mac.
3. When done, quit Disk Utility but don't restart your Mac.

7. Reinstall macOS

1. While still in macOS Recovery, choose Reinstall macOS, then follow the onscreen instructions. Learn more about reinstalling macOS from macOS Recovery. 
2. After reinstalling macOS, your Mac restarts to a setup assistant. Press Command-Q to shut down instead of continuing setup.