jQuery datepicker not showing on admin menu

Use admin_enqueue_scripts action instead of admin_head. Then see network tab in dev tools that datepicker scripts are included.

You need the include the jQuery UI CSS also, it is not included in WP core.

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