To do this task
Only the QuickBooks Administrator can do this task.
Open the items and inventory preferences.
Go to the Edit menu and click Preferences.
In the list at the left of the Preferences window, click Items & Inventory.
In the list at the left of the Preferences window, click Items & Stock.
Click the Company Preferences tab.
Select the Inventory and purchase orders are active checkbox.
If you want to avoid entering duplicate purchase order numbers, select the
Warn about duplicate purchase order numbers checkbox.