First of all check under which user this command is set to run in your Plesk Event Manager: Server -> Event Manager. This user should be able to run this command. On my 7.5.4 server I have this file with following permissions:
# ls -la /usr/local/psa/admin/plib/modules/sbm2/update_hosting.php
-rwxr-x--- 1 root psaadm 14729 Oct 17 2005 /usr/local/psa/admin/plib/modules/sbm2/update_hosting.php