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're about to create doesn't 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).
How to create a Purchase Order using the Units Entry Mode?
Click on the Purchase Orders tab > Add PO.
Click on the Standalone POs option.
On the right hand side you'll find the Actions button, click on it and click on Units Entry Mode to activate it.