■ Why is this necessary?
If the iCloud Calendar feature is turned off, the following problems may occur:
Your events will not be backed up, which can result in data loss.
Your calendar events will not be transferred when changing devices.
To avoid these issues, it is important to enable this setting.
■ How to sync with iCloud Calendar:
Open the Settings
app.
Tap on your Apple Account (your name at the top).
(If it shows "Sign in," please sign in first.)Tap on iCloud.
Tap Show All.
Turn on Calendars.
Please double-check the above settings.