command not found: createinstallmedia

No it should work, where did you get the Sierra installer app from

and are you using the correct Terminal command.

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

Did you download the InstallOS.dmg from the link provided in this article,

please use Safari whenever clicking on links.

How to get old versions of macOS - Apple Support

Scroll down to Download OS and click on macOS Sierra,

this will download InstallOS.dmg to your Downloads folder.

Open InstallOS.dmg to the InstallOS.pkg, open the .pkg

and you will see an installation window, this will create the

Install macOS Sierra.app you need to create a bootable USB

in your Applications folder.

When Apple released this version of Sierra they made a mistake in the coding that

will show an error when making a bootable USB.

The error will say Volumes/MyVolume (or whatever name you call your USB stick)

is not a valid mount point. As far as I am aware Apple has not bothered to fix this in over a year.

Use this workaround.

With Install macOS Sierra.app in your Applications folder open Terminal.

Paste in this command

sudo plutil -replace CFBundleShortVersionString -string "12.6.03" /Applications/Install\ macOS\ Sierra.app/Contents/Info.plist

press return

enter your password

press return

now paste in the createinstallmedia command for Sierra where MyVolume can be replaced with the name of your USB stick

If your USB name has more than one word separated by spaces you would need to type it in as

/The\ USB\ Name

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

press return 

follow the prompts

kmcmichael wrote:

Thanks Much!

Following your advice, I was able to finish creating the bootable installer for MacOS Sierra. Sadly, when I booted from it, it showed neither of the volumes on my internal hard drive. Not the main one on which Catalina is installed, nor the smaller one on which I hoped to install Sierra. Apple Support's only suggestion was to nuke the whole thing and start over.

macOS 10.13+ have changed the drive layout and file systems used on the boot drive so older operating systems like macOS 10.12 won't be able to work on (or with) an APFS formatted volume. While I know that macOS 10.12.6 itself is able to recognize an APFS volume the macOS 10.12 installer may not have this capability. You definitely do not want to install macOS 10.12 onto an APFS volume.

If you were previously running macOS 10.12.6+ on this system, then you should be able to access Internet Recovery Mode using Command + R to reinstall macOS.

Edit: removed need to fix installer as I missed @Eau Rouge command to fix the installer.