WordPress Custom taxonomy template

I think you are using brand taxonomy for the post type product.
You should create a file taxonomy-brand.php in your theme directory to display whatever you want in this page. You need not to create page template with this name, only create a file with this name in your theme. If you create any page by name brand please rename or delete it because page details will be open if you created by this name. If you want to display details of the product, create single-product.php in your theme directory.