Cannot close this order.
I understand what this message means, I’m just not sure if it is displaying legitimately.
This order was completely unpicked VIA Unpick Sales Order. When I look in Fulfillment Workbench, I see 0 across the board for Reserved/Picking/Picked.
Is there anything else I need to check I am not thinking of?
The first, but only the first, line and release show up in the PickedOrders table.
Is there a legit way to undo/remove this?
I was just about to ask if it showed up in Picked Orders.
I seem to recall having this happen once. I believe you can just create the pack and that will clear out the Picked Orders table. Then you can delete the pack, and close the release/line/order as needed.
Thanks @Chad_Smith. Something else is askew here. I tried that and I get this:
Part was in a PCID, and has since been unpicked/removed from the PCID. The PCID is indeed EMPTY but this record still exists with the PCID data in it. 
This got me to the solution. I had to do a bit extra because we use PCID. I had to re-reserve and release, then pick the line into the same PCID that was referenced before. After that I could create the pack from Picked Orders and delete it like you suggested.
Very on the wonk, as they say.