I have create a custom taxonomy called regions with acf. and have create also a select option on dokan multivendor dashboard where they can create new product

<?php
$regions = get_terms([
    'hierarchical' => 1,
    'show_option_none' => '',
    'hide_empty' => 0,
    'taxonomy' => 'region'
]);
?>
    <label for="regions"><?php esc_html_e('Region', 'your-text-domain'); ?></label><br>
    <input type="hidden" name="region_id" id="region_id" value="">
    <select name="regions" class="dokan-form-control">
        <option value=""><?php esc_html_e('Select Region', 'your-text-domain'); ?></option>
        <?php foreach ($regions as $region) {
        $parent_categories = get_terms([
            'taxonomy' => 'region',
            'hide_empty' => false,
            'parent' => $region->term_id
        ]);
        if (!empty($parent_categories)) { // check if there are any child categories
            ?>
            <?php foreach ($parent_categories as $parent_category) {
                $child_categories = get_terms([
                'taxonomy' => 'region',
                'hide_empty' => false,
                'parent' => $parent_category->term_id
                ]);
                if (!empty($child_categories)) { // check if there are any child categories
                ?>
                <optgroup label="<?php echo $parent_category->name; ?>">
                    <?php foreach ($child_categories as $child_category) { ?>
                    <option value="<?php echo $child_category->term_taxonomy_id; ?>" id="in-region-<?php echo $child_category->term_taxonomy_id; ?>"> <?php echo $child_category->name; ?> </option>
                    <?php } ?>
                </optgroup>
                <?php } ?>
            <?php } ?>
        <?php } ?>
        <?php } ?>
    </select>

and now i try to assign this post id when its created to regions like this

//path: dokan-lit/includes/Dashboard/Templates/Products.php

public function handle_product_add() {
    if ( isset( $postdata['add_product'] ) ) {
            $post_title     = sanitize_text_field( $postdata['post_title'] );
            $post_content   = wp_kses_post( $postdata['post_content'] );
            $post_excerpt   = wp_kses_post( $postdata['post_excerpt'] );
            $featured_image = absint( sanitize_text_field( $postdata['feat_image_id'] ) );
            <!-- i have added this line here to get regions id  -->
            $regionId       = sanitize_text_field($postdata['regions']);
            
            $product_id = wp_insert_post( $post_data );

                
                if ( $product_id ) {
                    
                    // and trying to assign this product id to region id like this.
                    wp_set_object_terms($product_id, $regionId, 'region');

}

But i see this is not working. Any idea how to fix this problem?