Start with the basics: Check all the accounts on the form to be sure none is missing the organization segment.
Try entering a value in Purchase Price Variance, OnHand Adjustment Account, and any other blank-but-enterable account fields on the form. Enter one of them, save, see if you still get the error. Then enter the next, save, check for the error, etc
You mention that the company has been created -- is it brand new? If so, log all the way out of Oracle then go back in and try again. Some key flexfields are read only when you log in, which means that if you make changes, they may not be seen until next time you log in.
If this is useful or resolves the issue, please use the buttons on the bottom of the response to mark it 'helpful' or 'correct' for the benefit of future readers with the same issue.