Error while trying to send multiple request (unresolved variable)

Question

I want to send different JSONs to this endpoint:

{{URL_API}}/products/

I need to update several information related to different products so i need to specify the product within the endpoint, i mean, i.e:

If you access this particular endpoint: {{URL_API}}/products/ you will get all the products but i need to specify the product that i want to update:

{{URL_API}}/products/99RE345GT

So, i decided to create a CSV file, i will update all the different product passing that file in the COLLECTION RUNNER screen, do you get me?.

For that, i created a new collection, i put this request_url:

{{URL_API}}/ns/products/{{sku}}

I edited the request body:

{
    "sku": "{{sku}}",
    "price": "{{price}}",
    "tax_percentage": "{{tax_percentage}}",
    "store_code": "{{store_code}}",
    "markup_top": "{{markup_top}}",
    "status": "{{status}}",
    "group_prices": [
        {
            "group": "{{class_a}}",
            "price": "{{price_a}}",
            "website": "{{website_a}}"
        }
    ]
}

all those fields between the {{}} will be completed by the CSV but it shows an error message over the url_request, {{sku}} seems to be wrong... it throws this message error:

unresolved variable, this variable is not defined in the active collection, environment or globals.

How can i solve this? I do not know what to do now.

What am i missing?