Error while trying to send multiple request (unresolved variable) - json

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?