Apple accounts can sometimes be a little awkward to connect to. There have been a number of changes from Apple over recent years.

The most important is that the require an “app-specific password”.

The second to check is that your calendar is accessible from iCloud.com.

To connect to Apple iCloud you will need to generate an app-specific password and use that instead of your regular one. They look something like this

You will need to enable multi-factor authentication for your account if you haven’t done so already.

Apple’s help article on how to do set this up can be found here: https://support.apple.com/HT204397

As of June 15 2017, Apple required that all applications use app-specific passwords when connecting to iCloud services. Therefore generating an app-specific password became mandatory when previously your regular password may have worked.

You should also check that you can access your calendar at https://www.icloud.com/ Sometimes Apple require some further verification before they make your calendar accessible through your iCloud account.