It is not mandatory to use that code. Indeed, many sellers do not use the app to run their seller accounts, and that is needed here for that code.
Seller needs to either go right of the item sold to More Actions, and there select Mark as Paid, and Mark as Dispatched.
Or, if you are on the Seller Hub page, go to Orders>> Awaiting Postage >> Print Postage Label, click on the item, and you will see those instructions top left of the page.
You may need a desktop browser, not a mobile browser, or an app to manage this.
