Upgrade failed

Hello everyone, i'm trying to install a localized language on my latest CE version 6.5.20 (build 1001), but averytime i use the upgrade fwizard, after pressing "next" button, the page returns me the following error and the task ends: Fatal error: Call to undefined function posix_getpwuid() in /volume1/web/sugarcrm/modules/UpgradeWizard/systemCheck.php on line 94
How can I fix it?
Thanks in advance.