So, two and a half years later and I've got myself into the same problem!
This time, I'd added a new supplier to create the Purchase Order for, and I'd missed the critical step of setting the Default Expense Category for the supplier, under the Accounting section for the new Supplier.
Once I'd cancelled the new PO, gone and edited the details for the new supplier I'd created, I could then create and save the new PO, and get my parts on order.
I hope that this helps as another suggestion for anyone else who ends up here when they're in a mess with Purchase Orders!
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