Apple Approval for Keychain
Apple has many security features within their iCloud Keychain. According to this support article, you do have some options:


You can use your iCloud security code that you set up when you started using iCloud Keychain. If you enter it too many times incorrectly, you will have to contact Apple support to try again. Too many incorrect tries will delete your security code from Apple's servers.
You can also verify your identity with a text message sent to the number used when first setting up iCloud Keychain. If you no longer have access to this number, you can also contact Apple support for more help.


Hope you can get this sorted out!