The problem was the way I was exporting my repository functions, I use:

export default {
  fetch,
};

Instead

export const fetch = () => {}

and in the test file I did this way to stub my repository function

import repository from "../src/repository/product.repository.js";

fetchStub = sinon.stub(repository, "fetch");

and now the Sinon can identify my fetch function, of course I had to refactor my controller too