What is the sudo command string to create a bootable usb drive for Sierra ?

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

The process takes around 30 minutes to complete, be patient, Terminal will look like it is not

doing anything but it is.

Thanks to Eau Rouge...

macOS Sierra bootable USB - Apple Community

See, How to create a bootable installer for macOS - Apple Support

Great news, take care my friend! :)