Hello, Nguyễn Đức Vương!
Boards are all or nothing in Trello. Either you can see every card, or you cannot see the board at all. This unfortunately means that as of this time, you cannot have cards that only certain people can see.
Something you may consider as a workaround is to attach either a Google Drive or Dropbox file that only the right people can see, and attach that to the card. Then, put all of the card information in that, instead. That way you still have that information somewhere on the board, but when clicked, only those with the right share permissions can actually open and read that document.
Also, feel free to request this as a feature by emailing feature-ideas@trello.com :)
Hope this helps!
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