In QuickBooks Point of Sale, you could end up in a situation where an item is created as a non-inventory part, has ended up with a Quantity on Hand, and the program will not allow you to change it to an inventory item.

You could try doing a return voucher for the 1 item, but that does not work.

You could try to do a Physical Inventory, or an Inventory Qty Adjustment, but these fail as it is a non-inventory part

In the Pro version and Multi-Store, there is something you can do.

Edit the item, then edit the Style.

In the Style edit, you can change the Quantity to 0, Save, then go back to the items list.

This should zero the Quantity, and allow you to edit the item to make it an Inventory Item.