Unable to turn on iCloud Keychain
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.