You must have write authorization for the calendar.
You can test write permissions on the calendar by connecting the calendar account to your iOS device and creating an appointment using the "+" in the calendar. If you don't have any permissions you will get a message.

You have to enable the Adhoc Booking feature.
Activate the Adhoc feature as follows: open the app -> open the menu -> scroll to "Ad Hoc Booking" -> switch on "Ad Hoc Booking" -> subordinate to the feature perform the configuration.