When entering a Vendor Bill you should always see items on the bottom of the "Add Invoice" screen to apply your invoice to. If you do not see anything in the bottom portion of the screen as shown below there could be a couple of reasons why as explained below.
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QB Mapping is unchecked - check this box and you should see items populate on the bottom portion of this screen.
Another reason you could not see anything to Apply your invoice to is the acknowledgement was added incorrectly. Double click on your acknowledgement and ensure you have actually acknowledged your lines.
Also make sure you have checked on "Vendor Sends Acknowledgements" in the Company Record.
