Events can only be deleted if no guests are assigned to it. You may need to remove the event from a Guest List Group/or temporarily un-assign all guests from groups that contain the event before being able to delete it.

The event must not have a menu or table plan saved either in order to be able to delete it.

Once there are no guests assigned to the event, choosing the to edit the event in the 'events coordinator' function,  will give you an option to delete.