iCloud Keychain Security Code?
How do I set up an iCloud Keychain Security Code?
iPad Air, iOS 12
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Do you use 2 factor authentication with your AppleID (you can check in settings, tap your account at the very top, then tap password & security and see if it is on)? If 2FA is enabled, then the iCloud Keychain passcode no longer applies and you can enable it on other devices using your AppleID password and a 2FA code.
From Get help with iCloud Keychain - Apple Support
“If you forgot your iCloud Security Code
If you turned on two-factor authentication, you don’t need an iCloud Security Code and the steps below won't apply to you. To check if two-factor authentication is turned on, tap Settings > [your name] > Password & Security on your iPhone, iPad, or iPod touch. Or on your Mac, choose Apple menu > System Preferences > iCloud > Account Details.
If you haven’t turned on two-factor authentication, reset the Security Code for iCloud Keychain or create a new Security Code.”