Shell_exec() warning

I got this warning, How can I deal with it?
WARNING [2020-02-22 01:13:31] 2455 /var/www/html/matomo/core/CliMulti/Process.php(264): Warning - shell_exec(): Unable to execute 'ps ex 2>/dev/null | awk '! /defunct/ {print $1}' 2>/dev/null' - Matomo 3.13.2 - Please report this message in the Matomo forums: (please do a search first as it might have been reported already)


Matomo tries to call ps ex 2>/dev/null | awk '! /defunct/ {print $1}' somewhere via shell_exec(), but while it doesn’t seem like shell_exec is disabled on your system, something is blocking PHP from forking subprocesses.