Yes, you are right, the accounts for the PO document for Inventory destination would be defaulted based on the setup of the inventory organization material account and operating unit.
The Account Generator workflow is responsible for defaulting all these information.
The error could be due to invalids of any procedure used in this flow, or when validation of setup of any of the accounting segments, etc.
Check if you have the same problem creating requisition for the same item, or check to see if you can post a manual journal entry for the account string that should be the charge account. These are to eliminate system or setup problem.
If all fail, best way to learn what the error might be is to gather a FND debug using note such as R12: How To Get The FND Debug Log For On-Line Processes And For Concurrent Requests (Doc ID 1320736.1)
The debug file could shed light on what might be incorrect, or where the error might be.
Hope this helps.