Mehdi:
Ubuntu is not a supported operating system for Ambari 1.6.1 and previous. You can find a list of supported OSes for Ambari in the “Automated Install with Ambari” guide. These are published at http://docs.hortonworks.com/
As of Ambari 1.6.1, the following operating systems are supported:
Red Hat Enterprise Linux (RHEL) v5.x or 6.x (64-bit)
CentOS v5.x or 6.x (64-bit)
Oracle Linux v5.x or 6.x (64-bit)
SUSE Linux Enterprise Server (SLES) 11, SP1 or SP3 (64-bit)