Cannot format "Macintosh HD"
You are getting this error because a newer version is installed on your mac the same thing happened with me but I was able to find a solution 

First you would need to download macOS Sierra Installer, now the problem is how to get it right, see if you go to the Purchased tab in the Mac App Store, you can redownload previously purchased macOS versions there (even if they are older than the currently installed one).

Then you would have to make a bootable disk containing the Install macOS Sierra.app

I am providing you a link to a YouTube video which will help you in creating the bootable USB stick link: https://www.youtube.com/watch?v=v4H5hKuI7yg&amp;t=130s

I am also adding the command required to create the bootable disk: 

sudo (path to the create install media file under installer.app&gt;Contents&gt;Resources) --volume {path to the USB stick (You are trying to make bootable)} —applicationpath (path to your Install macOS sierra.app)


e.g:

sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ macOS\ Sierra.app


Please make the necessary changes as per your files and volumes

And one more thing if you don't find the installer under purchased tab then you can download it from here : https://itunes.apple.com/cz/app/macos-sierra/id1127487414?mt=12

After you have successfully created the bootable disk you will need to press command+option or only option as soon as you power on your mac the next screen which appears will show you all the drives connected to your mac select the bootable drive and press enter after that you will be taken to a recovery mode where you can successfully format the new APFS formatted drive with the Old Apple Disk Formatting Version in order to install an older version of macOS!