This is an accepted solution.
Hello @David_Mil12 !
A completed Shopify POS order is marked in your Orders screen as Unfulfilled, you can't mark it as fulfilled from the Shopify POS app. You must log in to your Shopify admin to fulfill the order.
You can mark an order as fulfilled while it's still in the cart.
From the home screen of the Shopify POS app, tap More actions.
Under CART, enable Mark as fulfilled.
Tap Close and then continue with the order
Hope this helps!