I can't edit or delete a calendar event

On the Calendar tab, right-click the calendar in the left pane, Properties, and see if it's Read only.

For me, it seems to be the new calendar event dialog experimental feature. With it enabled, the event view is lovely, but there is no edit button. Turn that feature off and the edit button for the event is there as normal. Took me a while to figure it out - I checked the read-only which was not set, got concerned I'd totally broken something, then tried turning off that experimental feature in SETTINGS/CALENDAR.

Hope this helps others.