Apple requires third-party apps (like Calendars) to use app-specific passwords with iCloud accounts. An app-specific password is a single-use password you generate on the Apple ID webpage and paste into the password field while adding your account to Calendars.
So first, make sure to generate the app-specific password and add the account: https://support.apple.com/en-us/102654.
- Open Calendars.
- Tap the three-lines icon in the top left corner (skip this step on your iPad).
- Go to Settings > Accounts.
- Select iCloud and log in with your iCloud account credentials.
- Open the Calendars app.
- Click Calendars in the Menu Bar at the top.
- Select Preferences > + Add Account button.
- Click iCloud and follow the onscreen instructions.
Troubleshooting
If, after using the app-specific password, your account still can't be added, please check the following:
- The Internet connection on your device is stable. With a poor connection, Calendars might not be able to add your account.
- VPN is not enabled on your device. If you use a VPN, we can’t guarantee Calendars will work properly. The quality of the performance via VPN solely relies on the servers of the VPN network. For this reason, we recommend you turn the VPN off.
- There aren't issues with iCloud accounts on Apple's side. You can check this info on Apple's System Status page.