Please, try the following code,
namespace YOUR_PLUGIN;
function backEndAssets() {
wp_enqueue_style('MYPLUGIN_CSS_URL', plugin_dir_url(__FILE__) . 'assets/css/admin-styles.css');
wp_enqueue_script('MYPLUGIN_JS_URL', plugin_dir_url(__FILE__) . 'assets/js/admin-scripts.js', array('jquery', 'media-upload'), '1.0.0', true);
}
add_action( 'admin_enqueue_scripts', __NAMESPACE__ . '\backEndAssets' );
As you are not creating object inside plugin and want to load script then you must define namespace at the begging and define the plugin folder name and add this namesapce before function as I did. Either WordPress didn’t recognise the function.