If there is a the payment attached to the task, you need to remove it first by clicking Remove under the payment on the Transactions tab.
Remember, you cannot remove a payment that is already processing. You will have to wait until it is completed and request a refund from support.
On the task, screen click Edit
Click Archive Task
Note: We don't recommend archiving tasks if they were actually done. You can hide from your Action Centre simply by marking them as Completed.