If you want to allow a cashier to reprint a ticket, go to Configuration ‣ Settings and tick the option Allow cashier to reprint receipts.
You also need to allow the reprinting on the point of sale. Go to Configuration ‣ Point of Sale, open the one you want to configure and and tick the option Reprinting.
How to reprint a receipt?
On the Dashboard, click on More and then on Sales Orders.
On the orders list, search and click on the order you want to reprint.