How can I resolve synchronization issues with my Apple calendar?

"That account does not allow calendars to be added or removed"

"This account does not allow you to use calendars."

"Calendar does not have an account"

"Calendar has no source"

If you are experiencing any of the above issues or other problems with syncing with your Apple calendar, please check the following:

Open your device settings.
Go to "Calendar" → "Accounts".
Make sure your iCloud account is added.
If not, click "Add Account" → "iCloud" and sign in.
Go back one step to the "Calendar" screen.
Scroll down and select "Default Calendar"
Choose "iCloud Calendar".
Synchronization errors may occur if Gmail calendar is selected.
Return to the main device settings menu.
At the top, tap your "iCloud account" (where your name and photo are displayed).
Find the "Calendars" section and make sure the toggle is turned on.
Enter the "Apple Calendar" app.
Tap "Calendars" at the bottom of the screen.
Delete unneeded calendars.
Return to the "Smart Timetable" application.
Go to the calendar settings of your schedule.
Tap the “Sync” button in the upper right corner.
Done.