Cannot boot from usb
Try creating the USB Installer via Terminal.app


Download Sierra from the Mac App Store and leave it in your Applications Folder
Reformat your USB stick to HFS+ Journaled and name it Untitled
From Terminal copy and paste this 

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



That's it!