Howto Update SubItem

I tried to update subitem but somehow it failed using change_multiple_column_values

here is the detail

I list item first

{
  items(ids: [802335553]) {
    name
    board {
      id
    }
    group {
      id
    }
    column_values {
      id
      value
    }
  }
}

then the result of list item return 1 row


{
  "data": {
    "items": [
      {
        "name": "Hello World! 345",
        "board": {
          "id": "776547714"
        },
        "group": {
          "id": "new_group64730"
        },
        "column_values": [
          {
            "id": "subitems",
            "value": "{\"linkedPulseIds\":[{\"linkedPulseId\":802335556}]}"
          },
          {
            "id": "event_date5",
            "value": "{\"date\":\"1993-08-27\",\"time\":null}"
          },
          {
            "id": "remarks",
            "value": "\"hello\""
          },
          {
            "id": "date",
            "value": "{\"date\":\"2021-08-27\",\"time\":null}"
          },
          {
            "id": "link",
            "value": "{\"url\":\"http://www.facebook.com/eddyirwan\",\"text\":\"HELLO\"}"
          },
          {
            "id": "drafter9",
            "value": "\"ayam123\""
          },
          {
            "id": "deposit5",
            "value": "\"123.45\""
          },
          {
            "id": "numbers6",
            "value": "\"40.00\""
          },
          {
            "id": "deposit",
            "value": "\"23.45\""
          },
          {
            "id": "status31",
            "value": null
          },
          {
            "id": "status",
            "value": null
          },
          {
            "id": "courier",
            "value": null
          },
          {
            "id": "priority0",
            "value": null
          },
          {
            "id": "text4",
            "value": null
          }
        ]
      }
    ]
  },
  "account_id": 1791567
}

so i query available subitem inside 802335556

{
  items(ids: [802335556]) {
    name
    board {
      id
    }
    group {
      id
    }
    column_values {
      id
      value
    }
  }
}

and return one item

{
  "data": {
    "items": [
      {
        "name": "xxxx ko",
        "board": {
          "id": "776547722"
        },
        "group": {
          "id": "subitems_of_802335553"
        },
        "column_values": [
          {
            "id": "text8",
            "value": "\"hello world\""
          },
          {
            "id": "status",
            "value": null
          },
          {
            "id": "drafter",
            "value": "\"hello\""
          },
          {
            "id": "drafter6",
            "value": "\"deng\""
          }
        ]
      }
    ]
  },
  "account_id": 1791567
}

so i try to use change_multiple_column_values for this

{
  'query':
		mutation ($columnVals: JSON!)  {             
  		change_multiple_column_values (                   
    		board_id:776547714,                  
    		item_id:802335556,                 
    		column_values:$columnVals
  		)           
			{name}
  	}, 
	'variables': {
  	'columnVals': '{"drafter6": "pening kepala den teruk"}'
  }
}

and the respond

{
    "error_code":"InvalidColumnIdException",
    "status_code":200,
    "error_message":"This column ID doesn\'t exist for the board",
    "error_data":{"column_id":"drafter6"}
}

so i realise that, i dont have subitem id and i can’t update/change the subitem detail. Please advice.