So try that first part with mountain lion for a quick execution. Yes, that is because the installer application is in Mountain Lion but you messed up and wrote Sierra. Applications/Install OS X Mountain Lion.app does not appear to be a valid OS installer application. applicationpath /Applications/Install\ OS\ X\ Mountain\ Lion.app Confirm your flash drive is called "Untitled"
#Downgrade os mac sierra install
This part of code is to declare volume of the install media. Install OSX MountainLion.app is written as Install\ OSX\ MountainLion.app whereas Install OS X Mountain Lion.app is written as Install\ OS\ X\ Mountain\ Lion.app. If you haven't noticed space is called an escape character so every time you need space, you need "\ ".
If it is there, your command should be as follows: /Applications/Install\ OS\ X\ Mountain\ Lion.app\Contents\Resources\createinstallmedia Aren't you trying to do Mountain Lion? Why are you doing Sierra here? Anyway what's the name of the installer in the application folder? Is it Install OSX MountainLion or OS X Mountain Lion or sth else? After confirming that, right click (or double tap on trackpad) and do Show Package Contents, click on Contents then Resources and see if there is "createinstallmedia". This is trying to find create install media. The built-in Mac App Store mechanism will allow users to restore their Macs to macOS High Sierra, but no further upgrades are currently available through this method. Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia Can I upgrade my Mac from High Sierra straight to Mojave No. This is why people shouldn't just write the command. Looks like that guy thanked him before even trying it out. As you can see from the replies to that comment in the thread it's not compatible to use a installer of one OS with a different one and the command doesn't seem to work.