If you are using QuickBooks Online, you may not be able to see certain items in your Purchase Orders or Bill transactions.
This is especially true for Service and Non-Inventory items because, by default, they are not set up to be purchased from a vendor.
In order for the item to show up in a vendor transaction, you will need to enable the "I purchase this service from a supplier" checkbox and add in a cost.
NOTE: If you do not add in a cost, this may still not show up on the items grid.
Once that checkbox is checked, you will be able to see the item in your vendor transactions:
