Magento – Retrieve all available shipping methods/carriers via REST

I had the same problem.

Remove "Free Shipping" configuration because you don't need it (you already have "Cart Price Rules").

When your customer qualifies for free shipping it happens based on "Flat Rate" not in "Free Shipping".

We need to add custom_attributes:

$productData = [
        'attribute_set_id'  => 4,
        "type_id": "simple",
        "sku": "test-SKU",
        "name": "Test",
        "price": 100,
        "status": 1,
        'custom_attributes' => [
                ['attribute_code' => 'description', 'value' => 'Test Description' ],
                ['attribute_code' => 'short_description', 'value' => 'Test Short Description' ],
            ]
    ];

Payload:

{
    "product": {
        "attribute_set_id": 4,
        "type_id": "simple",
        "sku": "test-SKU",
        "name": "Test",
        "price": 100,
        "status": 1,
        "custom_attributes": {
            "description": "Test Description",
            "short_description": "Test Short Description"
        }
    }
}