Make sure that you have granted FirstSeed Calendar access to your calendar and reminder data.
On iPhone and iPad, open the Settings app and go to the Privacy section. Then, open both Calendars and Reminder sections and turn on the option for FirstSeed Calendar.
On Mac, open the System Preference and go to the Security & Privacy section. Click the Privacy tab and select both Calendars and Reminders from the left pane, and turn on the option for "FirstSeed Calendar" and "FirstSeed Calendar Mini" in the right pane.