In order to raise a purchase order for an Inventory listed product you need to create a supplier group that's linked to Inventory withdrawal. You then link your supplier to this group. Once linked when you raise the purchase order the full inventory list can then be viewed enabling you to select the product and raise the order.
To create your group go to Inventory > Suppliers > Supplier groups > new group
Within the General settings enter:
Enter a group number
Enter a group name, for example Inventory suppliers
Account - this should be your creditor control account, if you have a default chart of accounts this will be 2100
Number series - you can create a new number series for stock transactions or go with the number series for supplier invoices.
Within the Inventory settings enter:
Tick the box Access from Inventory
Tick each box below each section unless you have specified a nominal within your chart of accounts to capture the markups etc.
When you then create a supplier you wish to raise a purchase order for Inventory you must link the supplier to the Inventory supplier group you have just created.