On my purchase order I have it as 'open' at the top, but the line item is still closed, preventing the PO to show when I'm entering a bill. How do I untick the closed box
The Closed box should automatically be unticked if the Purchase Order is set to Open. Could you try marking the whole Purchase Order as Closed, then Save the Purchase Order? After that, mark it as Open and Save again. Let us know if the line item is still closed after following those steps.