When creating a Purchase Order (PO) and selecting an item associated with an income account, this message appears. Typically items for a PO are not associated with an income account.
If the selected item is not supposed to be an income item, you can edit the item and check the box Item performed by subcontractor...
Now you can assign an expense account with the item.
Check out what others say about this issue and review or add your own comments and solutions!