Postman integration with monday graphql APIs

i’m trying to create a new item in a board with populated columns using postman.
however when i submit the following mutation query as per the APIs guide, i keep getting the following error:{“error_message”:“Internal server error”,“status_code”:500}
i have tried to send it in different variations.
i did manage to create a simle item with no populated columns.

this is the query i use in postman:
mutation ($boardId: Int!, $itemName: String!, $columnValues: JSON!)



(board_id: $boardId, item_name: $itemName, column_values: $columnValues) 



**with the following graphql variables: **

"boardId": 1201957991,

"itemName" : "229",

“columnValues” : {

"name": "ido",

"address" : "PTV",

"phone" : "0504441814",

"order_date" : {"date" : "2021-04-11"},

"model" : "Marumba",

"__units" : 300



please find also screenshot attached:

your help will be much appreciated