What is the difference between default purchase cost and cost in the item's settings of the back office?
Which one will be taken by the system to calculate profits and margins?
If you specify a Default Purchase Cost for your item the cost field will automatically be filled when you create a purchase order with that item.
Otherwise, if you didn't specify the "default purchase cost" you will need to enter the Cost every time you make a new purchase order.
Other options that you can benefit of to quickly generate purchase orders are the Low stock and Optimal stock options. But this is a different topic, you can learn more here: https://help.loyverse.com/help/autofill-items-purchase-order