See the article https://support.apple.com/en-in/HT203783
Try setting up iCloud Keychain again. If you need to reset your Mac Keychain again, temporarily disable iCloud Keychain in iCloud Preferences before you reset your macOS Keychain.
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 mobile number 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 .