Failed to start samba due to a system error: Error running a command. cmd

@Umberto_Amante Welcome to the Rockstor community.

Your reported issue looks very much like what was reported in September 2017:

Take a look at that thread. Also very much worth subscribing to one of the update channels as 3.9.1-0 is now very old. But I think your reported issues was suspected as being down to an upstream interrupted update of some kind that left samba halt updated. That thread should cover it though along with some fixes that worked for those affected at the time.

Hope that helps.