Checkout API don't work

[ArtemShagrin]

When i send request to checkout.json, i get this object, why does it happens?I have registered all permissions for my app, but still see this

{
    "errors": {
        "line_items": {
            "0": {
                "variant_id": [
                    {
                        "code": "invalid",
                        "message": "is invalid",
                        "options": {}
                    }
                ]
            }
        }
    }
}