Authorise this Mac to use your data stored in iCloud
To access your data stored in iCloud, you must authorise your Mac to use your Apple ID.
If you’re asked to authorise this Mac by entering the passcode or password of a trusted device, enter the requested passcode or password. If you don’t remember it, you can select another trusted device that uses the same Apple ID account.
If you don’t remember the passcode or password of any of your devices set up with your Apple ID, you can finish signing in anyway but your data will be temporarily unavailable. To authorise your Mac so that you can access your iCloud data, open your Apple ID preferences, select iCloud in the sidebar, then select Keychain.
If you’re asked to authorise by entering your iCloud Security code, enter the code you created when you set up iCloud Keychain.
If you’re asked to authorise by approving this Mac on another device, go to any of your other devices set up with your Apple ID. Enter your Apple ID password to complete the approval so this Mac can use your account.