add_action( 'widgets_init', 'Y_widget_register' );
You have a typo – use underscores.
Here is the working piece:
function Y_widget_register() {
register_sidebar(array(
'name' => esc_html__('Sidebar', 'Y-07'),
'id' => 'y-sidebar',
'description' => 'Dynamic Right Sidebar',
'before_widget' => '<section id="%1$s" class="y-widget %2$s">',
'after_widget' => '</section>',
'before_title' => '<h2 class="y-widget-title">',
'after_title' => '</h2>'
));
}
add_action('widgets_init', 'Y_widget_register');