Option 1) Refresh your calendars and reminders from the Calendars app:
Open the Calendar app.
Choose View > Refresh Calendars.
Option 2) Try removing the account in Calendar > Preferences > Accounts.
Now add it back.
Option 3) If it still fails to work, try testing in Guest or new User.
CREATE A NEW USER
Go to System Preferences --> Create a New User in Users & Groups.
Switch to the New User by logging out/in or use Fast User Switching.
Login with Apple ID
Only select Calendar for this test.
Open Calendar and test by adding an event and see if it syncs with iPhone.
Do you still see the issue?
If yes, see these steps by Kappy to Fix Sierra Installation. I would suggest that you reinstall Sierra again.
If no, then the problem is in your User's folder.
Log out of the User then Log back into your user's folder.
With Calendar.app quit..
Go to the User's Library (see below)
Scroll down to Calendars
Move these folders to the Trash
- Calendars/Calendar Cache
- Calendars/Calendar Cache-shm
- Calendars/Calendar Cache-shm
Also in User's Library, scroll down to Containers. Move these to Desktop.
- Containers/com.apple.CalendarAgent
- Containers/com.apple.iCal
Scroll down to Preferences. Move these to Desktop.
- Preferences/com.apple.CalendarAgent.plist
- Preferences/com.apple.iCal.plist
Log out under the Apple in the Menu bar.
Log in
Open Calendar.app
Does it work now?
Show User's Library
The User's Library folder is hidden by default. To unhide: Select the Finder in the Dock. Under Go in the Menu bar > hold down the Option key and you’ll see the Library.
Finding the User Library Folder
http://www.takecontrolbooks.com/resources/0167/site/chap11.html#FindingtheUserLi braryFolder