iPhone 6 Plus cannot add calendars
You can't add calendars in the Calendar app, only in Settings. To add an event, you should have a calendar set up in Settings. 

If you do have calendars set up, make sure they are visible by going to the Calendars button at the bottom center in the Calendar app. Select "Show All Calendars" and see if you can now add events.

If you don't have a calendar set up, go to the Settings app, then iCloud, and make sure you enabled Calendars. Or you can add your own calendars from Outlook, Gmail etc. at Settings -> Mail, Contacts, Calendars -> Add Account.

Let us know if you succeeded. :)