"items" need to be an array and when you send in the POSt request, need to make it stringify. Call me if you get any issues.

order.json

{
    "token": "tok_mastercard",
    "user_email": "[email protected]",
    "full_name": "Chanaka De Silva",
    "order_note":"test description",
    "items": {{string_items}},
    "shipping": {
        "line1": "No 5B, Polwatta Road",
        "line2": "Gurugoda",
        "city": "Horana",
        "state": "Western",
        "postal_code": "12400",
        "country": "SL"
    },
    "billing": {
        "line1": "No 5B, Polwatta Road",
        "line2": "Gurugoda",
        "city": "Horana",
        "state": "Western",
        "postal_code": "12400",
        "country": "SL"
    }
}

## This is a JS code to make the item array 'stringify'. You need to send exact object. 
let item_array = [
             {
                 type: 'sku',
                 parent: 'sku_GXFKpYsKt9nHYZ',
                 quantity: 1,
             },
             {
                 type: 'sku',
                 parent: 'sku_GXFIWLDnYkVGWu',
                 quantity: 2,
             }
         ];
         
pm.variables.set('string_items', JSON.stringify(item_array));