A family of Microsoft relational database management systems designed for ease of use.
This isn't an Access error message; it's one that the developers of Northwind programmed into the button code. What I think it's saying is "You can't generate an Invoice for this order until all of the items in the order are Allocated"; if some or all of the items are Back Ordered (check your spelling!) then the program won't generate an invoice.
I'm not sure what you want to do - change the business rule? change the code? or what?