set-single-attribute-for-queried-products.php

<?php

$target_products = array(
     'post_type' => 'product',
     'post_status' => 'publish',
     'posts_per_page'=>-1
);

$my_query = new WP_Query( $target_products );

if( $my_query->have_posts() ) {

   while ($my_query->have_posts()) : $my_query->the_post();

     $term_taxonomy_ids = wp_set_object_terms( get_the_ID(), 'main', 'pa_sklad', true );
     $thedata = ['pa_sklad'=>[
       'name'=>'pa_sklad',
       'value'=>'main',
       'is_visible' => '1',
       'is_taxonomy' => '1'
     ]];
     update_post_meta( get_the_ID(),'_product_attributes',$thedata);
	 echo 'Updated product - '.$post->post_title.PHP_EOL.'<br>';

   endwhile;
}

wp_reset_query();
die();