Need help with a request

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I am developing a REST API in js. For this I have already made several get and post requests and everything is fine. In a get request which is specifically to get the variants of a product, the response of this request is the html of the password page. This does not happen to me with the other requests.
The url is fine and the request contains the Basic Auth.
I need to know if it should be sent in another way to enter that password protection or what should be done.
I clarify, this store is under development, so it is not yet transferable to a customer or a Shopify plan is not yet contracted.

The url that I send for this request is the following:

GET
https://{API Key}:{Password}@{store name}/api/2021-01/products/{id_product}/variants.json