I can't @ someone

HI ​@诶嗯嗯, thanks for reporting this! Could you please confirm whether the person you’re trying to @mention is directly invited to the board, or if they’re accessing it as a Guest or Visitor?

This will help us determine why you’re able to mention some users but not this particular person.

I just know other one can @mention the person,does it mean the person is directly invited to the board?