command not found, trying to create a bootable drive with Sierra for an older IMAC that has been wiped clean of OS

The steps mentioned by @Encryptor5000 are definitely required, but it is missing another very important step as well since the macOS 10.12 Sierra installer is broken when it comes to creating a bootable USB installer. The extra step involves modifying one character in one to the installer's file. See the instructions in the following link on how to fix the Sierra installer once you follow @Encryptor5000's Steps 1-3:

https://discussions.apple.com/thread/251386184?answerId=252696531022#252696531022

Then after fixing the installer, you can proceed with Steps #4 & 5 from @Encryptor.

FYI, your 2011 Mac can run macOS 10.13 High Sierra as well. This would be the best option assuming you are not using some third party audio software which won't work on High Sierra.

What model & year is the new iMac?

As Tesserax has suggest, have you attempted to reinstall the OS by booting into the Recovery mode? That would be the preferred way to accomplish this. Restart the mac while pressing and holding the Command-R keys (⌘R). Then select the Reinstall macOS app displayed on the screen.

Hi JoolaJen,

macOS Sierra comes in a disk image and must be unpacked before you can use it to create a bootable installer.

To unpack the installer:

1. Open and mount the installossierra.dmg file.
2. Double-click the installer package (.pkg) file inside the disk image.
3. Follow the onscreen instructions to unpack and install the macOS Sierra installer into your Applications folder.
4. Once the "Install macOS Sierra" app is in your Applications folder, use the command at the bottom in Terminal to create a bootable installer (based from your previous command).
5. Enter your password when prompted in Terminal. Terminal won't show any characters as you type the password.

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

You cannot create a bootable installer using a Mac that cannot run the OS you are creating the installer for.

In other words, if you’re trying to create an installer for macOS Sierra, you must use a Mac that can RUN Sierra.

What exact model is this "old" iMac? What version of OS X or macOS was it running before you erased its operating system? Have you even tried using Recovery Mode to re-install the OS?