When you need to create a Purchase Order using the Units Entry mode, it's necessary to do it trough the Standalone POs option, this means that the PO you are about to create does not have a corresponding Prebook.
This may happen in situations such as:
You need to purchase extra product for your open market sales.
You need to fulfill an order that had shipping issues or quality control problems.
You need to purchase the product for internal production (e.g.: bouquet operations).