Today I have a quick tip for you.If you’re looking for reason why you have the “Check In” button disabled\Grayed Out in SharePoint 2013, here is the quick explanation to that –
You probably have selected one or more documents from your document Library and looking to Check them In using the “Check In” button in the Ribbon.
If you have the above button disabled simply scroll through the entire library view and make sure that you have not inadvertently selected a file that was already checked in.
see Also : SharePoint 2013 End-User Tutorials
Original Link : Here