This article explains how you can manually open the cash drawer at any stage provided you have access to the Cash Up section
To manually open the cash drawer, go to Manager > Cash Up
In the Cash Up Screen, You will see the option Open Cash Drawer.
Click this button and your cash drawer will pop open.
If the cash drawer doesn't open after pressing this button, try disconnecting the drawer from either your computer or your receipt printer (whichever one it's attached to) then reconnect and try again.