If this error pops up for a center its MOST LIKELY due to the fact they added a "KIT" to their cart and didnt UNSELECT items of 0 QTY - to check that you will log in as the client - go to their cart and next to whatever KIT is in their cart you'll click edit and verify if thats the case - see pic BELOW
See how the top line is at 0QTY but it still says include in kit? You will unclick include in kit for those items and readd to cart - error will then be fixed. Then you can let the center know to check out as normal - see example TLE ticket for response. Obviously you may need to change the wording.
https://wmprinting.freshdesk.com/a/tickets/183098