/**
 * Create a product variation for a defined variable product ID.
 *
 * @since 3.0.0
 * @param int   $product_id post ID of the product parent variable product.
 * @param array $variation_data post ID of the product parent variable product.
 */

function create_product_variation( $product_id, $variation_data ){
    // Get the Variable product object (parent)
    $product = wc_get_product($product_id);

    $variation_post = array(
        'post_title'  => $product->get_title(),
        'post_name'   => 'product-'.$product_id.'-variation',
        'post_status' => 'publish',
        'post_parent' => $product_id,
        'post_type'   => 'product_variation',
        'guid'        => $product->get_permalink()
    );

    // Creating the product variation
    $variation_id = wp_insert_post( $variation_post );

    // Get an instance of the WC_Product_Variation object
    $variation = new WC_Product_Variation( $variation_id );

    // Iterating through the variations attributes
    foreach ($variation_data['attributes'] as $attribute => $term_name )
    {
        $taxonomy = 'pa_'.$attribute; // The attribute taxonomy
        $term_slug = get_term_by('name', $term_name, $taxonomy )->slug; // Get the term slug

        // Check if the Term name exist and if not we create it.
        if( ! term_exists( $term_name, $taxonomy ) )
            wp_insert_term( $term_name, $taxonomy ); // Create the term

        // Get the post Terms names from the parent variable product.
        $post_term_names =  wp_get_post_terms( $product_id, $taxonomy, array('fields' => 'names') );

        // Check if the post term exist and if not we set it in the parent variable product.
        if( ! in_array( $term_name, $post_term_names ) )
            wp_set_post_terms( $product_id, $term_name, $taxonomy, true );

        // Set/save the attribute data in the product variation
        update_post_meta( $variation_id, 'attribute_'.$taxonomy, $term_slug );
    }

    ## Set/save all other data

    // SKU
    if( ! empty( $variation_data['sku'] ) )
        $variation->set_price( $variation_data['sku'] );

    // Prices
    if( empty( $variation_data['sale_price'] ) ){
        $variation->set_price( $variation_data['regular_price'] );
    } else {
        $variation->set_price( $variation_data['sale_price'] );
        $variation->set_sale_price( $variation_data['sale_price'] );
    }
    $variation->set_regular_price( $variation_data['regular_price'] );

    // Stock
    if( ! empty($variation_data['stock_qty']) ){
        $variation->set_stock_quantity( $variation_data['stock_qty'] );
        $variation->set_manage_stock(true);
        $variation->set_stock_status('');
    } else {
        $variation->set_manage_stock(false);
    }

    $variation->set_weight(''); // weight (reseting)

    $variation->save(); // Save the data
}

$parent_id = 746; // Or get the variable product id dynamically

// The variation data
$variation_data =  array(
    'attributes' => array(
        'size'  => 'M',
        'color' => 'Green',
    ),
    'sku'           => '',
    'regular_price' => '22.00',
    'sale_price'    => '',
    'stock_qty'     => 10,
);

// The function to be run
create_product_variation( $parent_id, $variation_data );