Q: I can't add or change events in my calendar

Last Week I changed from Yosemite to El Capitan OS and since then, I get a window which says "You can't change events in the Found in Mail calendar" and this message  "Only the calendar administrator and any delegates with permission can make changes to read-only calendars. To make changes to the calendar, contact the calendar administrator.". I am the administrator but I don't know how to change the default to be a normal calendar where I can add or change appointments/events. I subscribe to one extra calendar and that also doesn't allow me to add or change events.