An error message appeared during day end processing "Location ___ does not exist for item ___" and then "day end processing could not complete all transactions."
This means that an inventory item is on a document (order, PO, or adjustment, for example) but the location that the item is referencing is not assigned to that item in Location Details. Therefore, Day end cannot process the transaction, and stops.
Go to Inventory Control / IC Items & Price Lists / Location Details. Enter the item and add the location that the day end is referring to. Now run the day end processing again. Once the DEP has completed, you can remove the location from the item, provided no more open documents have that item on them. You can check this from the Location Details screen, or by running the Item Status report from IC Stock Control Reports.