Brand it is not post meta of product but it is a term the same as Product categories, Product tags
Documentation of default wp function (which WooCommerce uses for getting product_cat, product_tag) -> https://developer.wordpress.org/reference/functions/get_the_term_list/
The function WooCommerce uses ‘get_the_term_list’ and we use ‘wp_get_post_terms’ are based on one default WP function -> https://developer.wordpress.org/reference/functions/get_the_terms/ where $taxonomy param for getting our brands of specific product is ‘brand’ as we wrote you before.
If you still need assistance please, provide us a screenshot of all possible options you see in this select -> https://imgur.com/a/mhEH4Sa.
There should be something like “Post Term” or if there is no such option then this importer does not provide such options and you may try to use another importer.
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