iCloud calendar sync no longer possible?

Hello there,

I would like to sync my iCloud calendar with Calendly so that Calendly knows when I already have appointments scheduled. However, following these steps, I encounter the problem that iCloud sync is not selectable anywhere. Only Google, Outlook, and Exchange calendar sync options are offered. The same issue arises under Integrations & Apps >> Calendars, where no iCloud sync can be found either.

Is there another way to do this?

Thanks
Oliver