Attempting to create a new item on a particular board but results in an error in the following JSON Data.

[ JSON Data ]

{
    "query": "mutation {\\n            create_item (\\n                board_id: ${boardId},\\n                group_id: \\"${groupId}\\",\\n                item_name: \\"${itemName}\\",\\n                column_values: \\"{\\"___9\\":\\"Monday \\"Create Item\\" Test\\",}\\"\\n            )\\n            { id\\n              board {\\n                id\\n                name\\n              }\\n              group {\\n                id\\n                title\\n              }\\n            }\\n        }"
}

[ Testing Environment ]

Using Client : Unirest
Using Language : Kotlin
Using Framework : Spring Boot

Please guide me on why the error occurs and what I should do to correct it.