[Solved] How to Fix macOS Calendar Error Couldn’t “move your calendars to iCloud because an error occurred”

Error “Couldn’t move your calendars to iCloud because an error occurred” prompt when macOS users try to open the Calendar app, this technical issue occurred at various mac OS versions including macOS Catalina. Detail error message as below:

Couldn’t move your calendars to iCloud because an error occurred.
Make sure you’re connected to the Internet, and then try again.

[Solved] How to Fix macOS Calendar Error "Couldn't move your calendars to iCloud because an error occurred"
[Solved] How to Fix macOS Calendar Error “Couldn’t move your calendars to iCloud because an error occurred”
Apply below resolution methods to fix this error.

Content Summary

Solution 1: Update macOS
Solution 2: Clear Caches and Calendar folders from Library

Solution 1: Update macOS

Step 1: Click on the System Preferences icon from the action bar.

System Preferences icon from the action bar
System Preferences icon from the action bar

Step 2: Click on the Software Update icon.

Software Update icon
Software Update icon

Step 3: Follow the on-screen prompts to install the latest available version if any.

Checking for updates.
Checking for updates.

Step 4: Click on the Update Now button and wait until the new version is installed.

Update Now button and wait until the new version is installed
Update Now button and wait until the new version is installed

Step 5: Restart your macOS.

Solution 2: Clear Caches and Calendar folders from Library

Reminder: Perform Time Machine Backup before proceed.

Step 1: Click on the System Preferences icon from the action bar.

System Preferences icon from the action bar
System Preferences icon from the action bar

Step 2: click on the iCloud.

iCloud in System Preferences
iCloud in System Preferences

Step 3: Uncheck the box associated with Calendars and Reminders.

Step 4: Use the action bar at the bottom to click on your Finder app.

Step 5: Press and hold on the Option key, then press the Go entry from the ribbon menu at the top and select Library from the list of available items.

Press and hold on the Option key, then press the Go entry from the ribbon menu at the top and select Library from the list of available items.
Press and hold on the Option key, then press the Go entry from the ribbon menu at the top and select Library from the list of available items.

Step 6: Double-click on the Caches folder.

Step 7: Press command + A on the keyboard to select all, then press command + Backspace to delete all selected items or right-click choose Move to Trash.

Select all items in Caches folder then move it to Trash.
Select all items in the Caches folder then move it to Trash.

Step 8: Return to the root Library folder, navigate to the Calendars app and repeat steps 6 and 7 to Select all items then move all the items to trash to clear the entire Calendar folder.

Select all items in Calendars folder then move it to Trash.
Select all items in the Calendars folder then move it to Trash.

Step 10: Restart your macOS.

Step 9: Go to System Preferences > iCloud and check the Calendars and Reminders.

Check the Calendars and Reminders
Check the Calendars and Reminders

Reference:
Apple Support > If your iCloud Contacts, Calendars, or Reminders won’t sync
Apple Support > iCloud: Advanced Calendar and iCal troubleshooting