Modern Calendars Filtering by Properties
The new modern experience calendars cannot use many of the functionalities standard list views have, for example filtering by properties.
The fact these filter options are still available, but just give meaningless error messages makes me think this is more of a bug than a suggestion, but after speaking to Microsoft support this is 'expected behaviour'.
The closest you can get is adding url parameters to add FIlterFields, but these views can not be saved and only provided as links for users.
Please allow all the great functionalities within list view settings to work in modern experience calendar views!