exec(): Unable to fork [whoami] message

Hi Guys,

I recently downloaded query monitor for my current site as I kept getting the 03 service unavailable message frequently. could someone please explain what the following PHP error message means please?

wp-content/plugins/query-monitor/collectors/environment.php:291
exec()
wp-content/plugins/query-monitor/collectors/environment.php:291
do_action(‘shutdown’)
wp-includes/load.php:1007
shutdown_action_hook()
Unknown location