In order for candidate.fyi to access the calendars you have access to in Google Calendar, you need to currently be subscribed to it.

The calendars you are subscribed to will appear on the left side in Google Calendar like below:

If you don't see a calendar that should be there, click on the plus button next to other calendars and select "Subscribe to Calendar"

You can then search for the calendar you're looking to subscribe to.

Once you've successfully subscribed (and you see it now populated in your list of calendars), go back into candidate.fyi and refresh the calendar list!