Why do I see the Completed work orders not closed error on period close validation

The period end validation alerts you about completed work orders that aren't in the Closed status in Manufacturing. This validation is applicable for process and discrete manufacturing work orders and not for maintenance work orders.

You can identify the work orders from the Manage Work Orders page by searching for completed work orders for the date range of the period. If you find any work order that can be closed, close the work order and try the validation again.

If you don't close the work orders, the product cost adjustments in the case of actual, perpetual average, and periodic average cost methods and variances in the case of standard cost method might get created in a different period than the product completions. On the Manage Work Orders page, you can close the work order with the current date or you can backdate it to any date after the last transaction reported for that work order.

Also, you can ignore this validation because it's only meant to inform you about work orders that aren’t closed in Manufacturing.