There is no ' Mark as Collected' the only way to show this is to enter / scan , that collection code which the buyer should bring with them.
Send the buyer a message to send you this code, explaining it facilitates a faster release of your payout for the item. They can find the code either in their emails ( worth checking Spam too ), their eBay messages or in their order details.
You can then enter those numbers manually where it says Confirm Collection.
If they don't , or cannot find this code, you will need to wait a minimum of 14 days for the payout.
For future collection listings, worth sending the buyer a message after purchase, reminding them to bring that code, otherwise the item will ne be released. Or, write the reminder in the actual listing.
@mick_e_29