Before you can create a purchase order you must first have your storage locations and vendors added to VetLogix. Check out these articles for how to set those up:
VetLogix recommends adding inventory via Purchase Order, both for initial inventory counts as part of a new practice setup, and every time new inventory is added and/or intended to be tracked in VetLogix.
This video will walk through the first part of purchase orders, which is creating them using the desktop admin app (app.vetlogix.io).