[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.

Apply below resolution methods to fix this error.

Solution 1: Update macOS

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

Step 2: Click on the Software Update icon.

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

Step 4: Click on the 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.

Step 2: click on the iCloud.

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.

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.

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.

Step 10: Restart your macOS.

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