Material Inventory is not available in this version of OCTO Ops. The Coming soon badge on this page refers to the Material Inventory feature itself, not just to this guide — there is no Material Inventory list, form, or profile you can open in the application yet.
Material Inventory still appears in the left navigation under Production, but selecting it opens a page-not-found error. If you're looking for the materials used in production, see Materials instead; general stock levels are covered in the Warehouse guide.