Quik Invoice will read the IC Item by Location file to determine if that Item is allowed to be at the location. If that permission is set to false, Quik Invoice will not allow you to ship that IC item from that location. You must select a different location on the Invoice detail screen, or go into IC and allow that Item at that location.
Because the Location code is blanking out for you, this means that this Item has not been set to be Allowed at any IC Location.
Quik Invoice is solely relying on the information stored in IC for this. Any errors encountered due to invalid locations are because the IC locations and permissions are not set correctly. Edit the location in IC and choose 'Select Items'. Make sure that all items have an arrowhead next to them. If you want all items allowed at the one location, the easiest way is to use the Range function on Edit Locations and Permit = Y for all items.
I do not believe setting the 'Allow Items at all locations' flag in IC is enough to allow Quik Invoice to proceed; the IC location must be set up.
Regards,