As a Responsible User of a Task, if you are trying to sign off on a Change Request Task but you do not have the option to close the task, it is most likely because the Change Request has not yet reached the Implementing status.
While a Change Request is in the Implementing status, you will have the options to close the task from the Actions dropdown in the Change Request Details Page:
Relevant for: Administrators / Managers / Regular Users / Read-Only Users.
Please note: Only the users responsible for executing the Change Request Task will have the Close Task icon in the Actions dropdown.