iCloud Keychain not working properly

Two factor authentication account is turned as on in iPhone but not in your Mac , the advanced section will not be shown when 2FA is turned as on in iPhone this is a basic difference between secondary authentication account and 2FA .

You have to turn on 2FA from Mac Two-factor authentication for Apple ID - Apple Support

The option button in front of keychain from Mac will not be shown using 2FA account .

No need to remove the devices if they are verified from Mac , Use two-factor authentication for Apple ID security on your Mac, iPhone, and iPad - Apple Support

If you already use two-factor authentication, you can no longer turn it off. Certain features in the latest versions of iOS, iPadOS, and macOS require this extra level of security. If you recently updated your account to turn on two-factor authentication and then decide not to use it, you must stop using it within 2 weeks. Open your enrollment confirmation email and click the link to return to your previous security settings.

Did you clicked the link from mail application and reverted back to secondary authentication account ?

If iCloud Keychain is corrupted in Mac .

( a ) Use an iPhone and sign in with same Apple ID and password in your Mac .

( b ) In iPhone go to settings , iCloud open keychain and click on advanced two options will be shown reset code and reset iCloud Keychain , click on reset iCloud Keychain .

( c ) A verification code will appear on your iphone through sms and it will prompt to generate a code , think of code and fill up in the field so keychain will be set up in iPhone , write down this code .

( d ) Come to Mac , in system preferences > click on iCloud the box of keychain is unchecked , so tap the keychain box , it will show waiting for approval click on option button , enter the passcode that is generated in iPhone , click on confirm .

( e ) Again in iPhone a pop up will appear showing approve or deny for name of Mac to approve for keychain , click on allow .

( f ) In Mac click on option button enter trusted number an sms will come having a verification code , enter it the trusted number will be saved and visible and finally keychain is set in Mac .