What prints when I cancel an order? My kitchen kept cooking a cancelled one.

Two things now print the moment an order is cancelled or voided, from the POS or from the orders board. Every station that got a ticket for it prints a CANCELLED slip with the dishes and the reason, so nobody cooks it, and the counter prints a CANCELLED receipt for the file. Settings → Printing & receipts shows both under When an order is cancelled. The POS and the counter tab in the docs covers cancellations on paper.