It's not possible for agents to manually close a ticket, or select Closed as a status within the ticket. This is because closed tickets cannot be modified in any way, and triggers, automations, or the Zendesk API sometimes make updates to the ticket upon (or after) ticket solve.
However, you can use triggers to automatically close tickets based on certain conditions, for example, the presence of a particular custom field value, or a particular tag. See below an example workflow.
If needed, create a view of the tickets you want to close. You can work through this view to add the tag to your tickets - either by updating each ticket individually or by using the bulk ticket editor to add the tag to all the tickets on the page of the View.