A Salesforce administrator updating a record page to add a Chatter component to the Action...

Feed tracking has to be enabled for the Action Plan object in order to add a Chatter component to the Action Plan Lightning record page. Feed tracking allows you to see updates on records in Chatter feeds. You can enable feed tracking for custom objects, such as Action Plan, from the setup menu. Once feed tracking is enabled, you can add Chatter components, such as Feed or Publisher, to the record page using the Lightning App Builder.References: [Feed Tracking], [Chatter Components]