Niki:
I got this same error when trying to edit orders for products that didn't require a shipping address such as virtual orders. I temporarily resolved this Warning error by editing the edit_order.php file from Edit Order ...
I would caution against disabling warnings output by Edit Orders. Warnings are usually displayed for well thought out reasons. The one you mention exists to indicate the order details (specifically all of the addresses) are not fully populated in the database (or may contain invalid information). It goes on to let you know the missing / incorrect data may cause problems with some order total modules and tax calculation. This specific WARNING is only a warning and does not indicate a failure within Edit Orders.
You do bring up a good point however. Perhaps this warning should be split into two parts... One for missing information and a second one for invalid data. This would allow us to emit more accurate "how to fix" information. Or perhaps "Edit Orders" should just automatically copy the billing address to the shipping address fields when no shipping address is entered...
Some of the reasons for this warning message:
A handful of the order total modules for Zen Cart attempt to read addresses (such as the shipping address) and can crash / emit invalid totals when various address fields are not populated (or contain invalid data). When this address data is not present and one of these order total modules enabled, editing an order may result in corrupted order information (and pricing) in the database.
When a new order is created (on behalf of a customer - such as email or phone orders) directly in the administrative interface, this also serves as a reminder to the person entering the order to fill out all of the address fields which may be required by various order total modules.
For example, what happens if for some reason the order needs to have one of the products shipped? While a rare case, a customer may call in and ask instead for a physical copy and to have the physical copy shipped. Editing the order to add shipping will cause problems with pricing and tax if the billing address is not present
So even when a purchase may be for "virtual only" products, if you are going to edit the order, entering the shipping address (populating with the billing address) is usually a good idea.
NOTE 1: just as a reminder, Edit Orders should ONLY be used to edit customer details / addresses, products in the order, and order totals. It should not be used simply for changing the order status. If only changing the order status (or adding comments), it is always better to use the "details" button instead.
NOTE 2: If one reads the very next post after the one quoted by Niki, one will notice the actual cause of the problem for the OP (Edit Orders requires ot_subtotal enabled). The current versions of Edit Orders display a warning when ot_subtotal is not enabled.