When I enter Y after the prompt, I get a "Command Not Found" message. What do I do?

Obviously you are trying to make a bootable flash installer for Monterey, right?

Do you have "Install macOS Monterey.app" in /Applications? If you have macOS Extended (GUID) formatted "Untitled" USB flash drive on the Desktop, then this Terminal command should work and prompt to erase the drive and also make "Y" prompt at some stage. (You can use other names for the flash drive but it must be the same as in the end of the command).

sudo /Applications/Install\ macOS\ Monterey.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled

How to create a bootable installer for macOS - Apple Support

Jeffcisp wrote:

When I enter Y after the prompt, I get a "Command Not Found" message. What do I do?

?

What command...

y is not a command, if you are in dialog with the shell, y might be a Yes if it is asking for conformation to continue: y/n

Jeffcisp wrote:

Thank you! The Apple Help page had an incomplete Terminal command, leaving off the application address part.

Actually it doesn't…

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

Glad you got it working.

Just to add, that message indicates the Command you typed in is incorrect.