Unable to turn on iCloud Keychain
Solution 1:
You'll need to approve the iCloud activation from another device. It may pay to check your Apple ID has a valid phone number associated, so you can receive an SMS verification.
From Apple Support https://support.apple.com/kb/HT204085:
If you can't access any of your other devices that are using iCloud Keychain, you can still set up iCloud Keychain on another device if you have these items:
Your iCloud Security Code
The device that is using the SMS-capable phone number you provided when you first set up iCloud Keychain. A verification code is sent via SMS to this phone number. If you can't access this number, contact Apple Support, who can verify your identity, so that you can complete setup on your new device.
After you complete the setup, your iCloud Keychain will be pushed from the cloud to the new device.
If you use two-factor authentication with your Apple ID, enter your password and the six-digit verification code that's automatically displayed on your trusted devices and authorize the new device. Then turn on iCloud Keychain.
Solution 2:
Following Apple support advice solved the problem:
- Reset network settings
- Logout and login
- Reset all settings
Solution 3:
iOS 15
After login into icloud, you can be prompted about Merge or Do not merge data. I think if you choose "do not merge", you will not be able to turn on Keychain.
After logout icloud -> reset all settings -> login and choosing merge, then the Keychain was turned on.