验证器做数据唯一性验证一直报错，是不支持验证还是我写的有错

  $res = $request->post();
        //数据验证
        $validator = Validator::make($res,[
            'username' => 'required|string|unique.username',
        ]);
        if ($validator->fails()){
            return $this->ajaxReturn(0,$validator);
        }