Maybe the path to landing_page.php is just wrong ?
render_template
(String) The path to a template file used to render the block HTML. This can either be a relative path to a file within the active theme or a full path to any file.
// Specifying a relative path within the active theme
'render_template' => 'template-parts/block/content-testimonial.php',
// Specifying an absolute path
'render_template' => plugin_dir_path( __FILE__ ) . 'template-parts/block/content-testimonial.php',
