In Salesforce Classic mode, the close case button takes user to a special “close case page”. The status on this close case page only displays the close statuses. It also has separate fields that are required and only visible on the close case page.
This functionality is missing from Lightning experience.
Astrea Case Close is a free Lightning Component available on AppExchange. As the Close case button and the close case page layout are not available in lightning yet, this component is used to close the cases in the Salesforce. This component basically mimics the functionality of Salesforce Classic of providing a separate layout for closing cases. It has point and click graphical user interface which is simple and easy to use.
The full details of the setup can be seen in the User guide available on AppExchange and from here
You can download the component from Appexchange.
Before using the component, System Admin will need to add the “Case Close” button on the Case page layout, as shown above. For changing the status of the cases to “Closed”, select CaseClose quick action and enter the case information.
The screenshot below displays the Close Case page that will allow user to enter comments, Status and Close reason, and then close the Case.
Select the values for Status and Case Reason as they are the required fields here. After that click on Case Close button. While closing the case, user can also enter the internal comments as an optional field