So you are getting an error in AR that an inventory organization does not exist. You should be able to work around this by creating an inventory organization and attaching it to Order Management Parameters.
Please verify that the Order Management responsibility you were using is pointing to the right operating unit. May be that is why you could not see the inventory organization.
Also verify if you have completed the accounting section of inventory organization setup. You will not be able to use your inventory org until accounting setup is done.
If this still does not work please post the images of your inventory organization setup and the site profile option of mo:operating unit. Thanks.