Source: https://www.prestashop.com/forums/topic/276384-solvedunable-to-modify-customer-order-from-backoffice/?tab=comments#comment-1388383
Back Office—->Orders—>Statuses—>Preparation in Progress(Or any status which is set as default), I unchecked “Show Delivery PDF.”
Status should not have “delivered” option.
In table ps_order_history remove this order’s “Delivered” state
https://www.prestashop.com/forums/topic/276384-solvedunable-to-modify-customer-order-from-backoffice/#entry1388383
(Not sure if necessary, should test)
In tables ps_orders/ps_order_invoice
delivery_number change to 0
delivery_date change to 0000-00-00 00:00:00