You are unable to check availability of other users in the App Calendar in Mac OS BigSur. All users are shown as available, even if they are marked busy.
This issue could be arising due to Apple modifying its CalDav implementation in the Big Sur release.
Please note that an enhancement request to add support for the Show Availability functionality over CalDav for Big Sur has already been raised to the Development Team for consideration in a future release. You can track the same through the Kerio Connect Release Notes page. Please do note that an ETA is not available for this request.
In the meantime, you can set up Apple Calendar using an Exchange connection instead - the Show availability functionality works over EWS in the Big Sur release. For guidelines, please refer to Setting Up Exchange Accounts on macOS. Note: In Step 7, you should only select Calendars, if you only wish to have Calendars set up over EWS.
Alternatively, you may use a 3rd-party Calendar application like BusyCal for Mac - the Show Availability functionality has been tested to work on it over the CalDav protocol.