Authorise this Mac to use your data stored in iCloud
To keep your protected iCloud data secure, you must authorise this Mac to use your Apple ID account.
If you’re asked to authorise by entering the passcode or password of an iOS device or a Mac you set up for iCloud, enter the requested passcode or password. If you don’t remember it, you can select another device that uses the same Apple ID account.
If you don’t remember the passcode or password of any of your devices set up for iCloud, you can finish signing in to iCloud anyway, but your protected data will be temporarily unavailable. To complete the approval so this Mac can use your protected iCloud data, open iCloud preferences, 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 for iCloud. There, you’re asked to enter your Apple ID password to complete the approval so this Mac can use your account.