Add Attribute to WooCommerce Product Programmatically

<?php
$attribute_name = 'pa_color'; //slug of the attribute(taxonomy) with prefix 'pa_'
$attribute_value = 'green'; //slug of the attribute value (term)

$product_ids = []; //array to hold Product ID(s)
//getting all simple products
$args = ['post_type' => 'product', 'posts_per_page' => -1];
$loop = new WP_Query($args);

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

    global $product;
    $flag = 0;
    //get product attributes
    $attributes = $product->get_attributes();

    if (empty($attributes)) {
        $product_ids[] = get_the_ID(); //product which does not have any attributes
    }
    //if it has product attributes
    else {
        //looping throught all the prodct attribute
        foreach ($attributes as $attribute) :
            echo $attribute['name'];

            if ($attribute['is_taxonomy']) {
                if ($attribute['name'] == $attribute_name) {
                    $flag = 1;
                }
                //to get the value attribute
                //$values = wc_get_product_terms($product->id, $attribute['name'], ['fields' => 'names']);
            }
        endforeach;
        if ($flag != 1) {
            $product_ids[] = get_the_ID(); //product which does not have the given attributes
        }
    }
endwhile;

wp_reset_query();

<?php
foreach ($product_ids as $product_id) {

    //Appending term to the object/product.

    $term_taxonomy_ids = wp_set_object_terms($product_id, $attribute_value, $attribute_name, true);

    $data = array(

        $attribute_name => array(

            'name' => $attribute_name,

            'value' => ",

            'is_visible' => '1',

            'is_variation' => '0',

            'is_taxonomy' => '1'

        )

    );

    //First getting the Post Meta

    $_product_attributes = get_post_meta($product_id, '_product_attributes', TRUE);

    //Updating the Post Meta

    update_post_meta($product_id, '_product_attributes', array_merge($_product_attributes, $data));

}