To sync phone calendars, you must enable calendar permissions:

1. Head to the Home (🏠) or Calendar (📅) tab 
2. Tap the cog / settings icon (⚙️) in the top right corner
3. In the 'Your calendar' section, tap 'Sync calendars'. If you have not given access, a popup will show up. Tap 'Go to Settings' to go to your device settings.
4. Locate 'Calendar' permissions and give Howbout 'Full Access'.