Can I install Mac OSX Sierra on USB drive?
You certainly can install macOS Sierra to a USB device. I would advise you use USB 3 or 3.1 in order to ensure a smoother experience while booted from this device.

First of all, use the Mac App Store installer for macOS Sierra, which you can get here: get macOS Sierra. Learn more about obtaining macOS Sierra here: How to download macOS Sierra

Next, it's best to format your USB device as Mac OS Extended (Journaled). With Scheme as : GUID Partition Once you do this, open the macOS Sierra installer. When you are prompted to choose an install destination, make sure to choose your USB device.