In Odoo v16 Community Edition, the Projects module uses Kanban views to manage tasks. To mark a task as "Closed" you can do the following:

1. Go to the Tasks list view and open the task that you want to mark as "Closed".

2. In the task form, you will find a field called "Stage" that shows the current stage of the task. To mark the task as "Closed", you can change the stage to "Closed" using the drop-down menu.

3. Once you set the stage to "Closed", the task will be removed from the Kanban view, and it will be visible in the "Closed" filter.

4. You can also move the task to the "Closed" stage by dragging and dropping the task card to the "Closed" column in the Kanban view.

5. Once you have closed the task, it will no longer be available on the task list view. You can check the closed tasks by clicking on the "Closed" filter or "Closed in the last 7 days" filter.

It's important to notice that the stages are defined in the project settings and you can customize them to adapt to your workflow and business process.

Also, it's worth mentioning that you can use the "Task" menu and select "Closed" to see all closed tasks, and you can also use the search bar by typing "Closed" to filter the closed tasks.