When you see "This calendar is read-only. To edit, please ask the administrator." on the event edit screen, it means the event is shared via Google Calendar and you have not been granted edit permission. In this case, you cannot edit or save the event from aircal.
aircal integrates with Google Calendar to display and edit events.
Whether you can edit an event depends on the calendar's sharing permissions in Google Calendar.
This is not aircal's own permission system: if the calendar is shared with you as "view only" on Google Calendar, you cannot edit it.
1) Ask the Google Calendar administrator to grant you edit permission
Ask the administrator of the Google Calendar to change your permission to "Make changes to events" or "Make changes and manage sharing".
Q. Is this an aircal bug?
No. This is a restriction based on the Google Calendar sharing / permission settings.
Q. Why can't I save changes?
You have "view only" permission on the Google Calendar, so you cannot change events.
Q. Where can I change the permission?
Google Calendar sharing settings can only be changed by the administrator.