Let me explain this myself so people won’t make the same mistakes.
When I saw the last line of the error message
Error: Xcode alone is not sufficient on Sierra.
Install the Command Line Tools:
xcode-select --install
My thought was: I already have Xcode why the system ask me to “reinstall” it. However, thanks for @SamiKuhmonen @ Beartech @patrick kuang suggestion, I search a page (in Mandarin). xcode-select --install does not reinstall the whole Xcode. It means install some missing command line tools which is required by installing Ruby.