How to display registered customer list as dropdown in WooCommerce checkout?

Woocommerce customers are just regular WordPress users, so you can use the standard get_users() function.

You can pass the argument role to that, so something like…

$customers = get_users( 'role=customer' );

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)