When I try to add an event in the Calendar “account is read only” - how do I change?
You cannot add or edit events in a read-only calendar.
When saving the event, choose a calendar that you have write permission to.
That will be a Local Folder calendar, as in the screenshot above, a calendar that has been added as part of your email account (like GMail or Outlook.com), or a calendar that you have added as a separate account.
If you added your calendar as an Internet Calendar, as in the following screenshot, that will always be read-only, so you cannot add or edit events.