php-fpm failed to start after installtion

Hi,

I'm new to OMV and I just installed the latest version following the "Debian Operating System", I'm running Debian 9 on Intel NUC.
I cannot get to Web GUI to work, I'm getting 502 bad gateway from nginx, and when I'm looking into monit I can see that:
1. php-fpm failed to start
2. nginx failed protocol test
3. omv-engined process is not running

Please help.

BTW, I have also noticed the following error when running omv-initsystem
root@NAS:/var/log# omv-initsystem
PHP Fatal error: Uncaught Error: Call to undefined function mb_strtolower() in /usr/share/php/openmediavault/autoloader.inc:28
Stack trace:
#0 [internal function]: {closure}('OMV\\System\\Stor...')
#1 -(3): spl_autoload_call('OMV\\System\\Stor...')
#2 {main}
thrown in /usr/share/php/openmediavault/autoloader.inc on line 28