At this moment, Required Actions are not meant to be completed. The purpose of this area of Core iQ is to set perpetual reminders to complete tasks until the Opportunity is "won" or "lost". Therefore, in order to complete a Required Action, you need to schedule another one in its place. No Opportunity should be without a Required Action of some sort. If you are worried about the counter on your Core iQ dashboard getting too full, Required Actions drop off your Dashboard after the number of days of inaction specified by the "Set Communications Past Due After Days" field located in Bank/Credit Union & Core Import setup under the Application Settings tab.
The default setting for "Set Communications Past Due after Days" is 5, but can be adjusted if needed. Adjusting this field will also impact the Past Due metric for communications. The exact time that the item drops from the list of 'Required Actions' is dependent on the date/time noted on the required action and the date/time that the dashboard is being viewed. Please contact support@onovative.com if you have questions about this functionality.