calendar
I am unable to access Calendar on iCloud.com and it is not syncing with my iMac. Other apps such as Photos are accessible and synced.
iMac Line (2012 and Later)
What happens when you try to access Calendar on iCloud.com ? Error message(s)?
Try going to System Preferences/iCloud (Apple ID for Catalina or later) and stop Calendar syncing by unchecking the box. Wait a couple of minutes, then check syncing. Allow time for the sync to complete. The photos sync is extremely slow. It could take several days or more for the photos to sync..
Last resort - have you tried signing out of iCloud, restarting, and then sign back in?
See the Apple Support article below.