Getting 404 when creating a conversation

I’m trying to create a conversation with https://api.intercom.io/conversations

REQUST BODY:

{
    "from": {
        "type": "contact",
        "user_id": "123456789"
    },
    "body": "Hello"
},

HEADER PARAMETERS:

Intercom-Version: 2.10

Authorization: Bearer <access_token>

RESPONSE:

{
    "type": "error.list",
    "request_id": "0002sf5fchfdlt2eus20",
    "errors": [
        {
            "code": "not_found",
            "message": "User Not Found"
        }
    ]
}

Could I be missing something?