how to I know my iCloud security code? I get four blank space to fill it. The question is "Your iCloud data is protected by your iCloud security code". But I don't know what is the code. some help me?

See this -- If you enter your iCloud Security Code incorrectly too many times - Apple Support.

How do you find out your security code for iCloud?

System Preferences/iCloudKeychain/Options and click Change Security Code.

Make sure that you disable two-factor authentication in macOS Sierra. Otherwise you won't get the chance to reset the Security Code (as indicated by the documentation)

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