it appears that the calendar only syncs with systems such as Google and Outlook. If you rely on an iCloud calendar like I do, you are out of luck.
eM Client does normally sync with iCloud Calendars.
When you add the account via the “automatic account wizard”, make sure you select / tick “Calendar in Services on the last screen” as per the below eM Client with iCloud info / setup page below.
(eM Client with iCloud) - Mail , Calendar & Contacts
https://www.emclient.com/em-with-icloud
Also see @Gary post in the following thread how to manually add an iCloud calendar.
Lastly you have to setup an “app specific password” if you haven’t already for your iCloud account, to then sync it with eM Client. See the following eM Client support page