Where is the inittab file?

If you do man inittab in your terminal, all shall be revealed:

   The  /etc/inittab  file was the configuration file used by the original
   System V init(8) daemon.

   The Upstart init(8) daemon does not use this file,  and  instead  reads
   its  configuration  from  files  in  /etc/init.   See  init(5) for more
   details.

Back in the days the "System-V" init service was used in Ubuntu, and it used the /etc/inittab file.

Some time ago (around 2006) the "Upstart" init service replaced SysV. During these days you could follow the top answer and use man inittab to get info on this change.

At the time of writing (e.g. for Ubuntu 16.04) the "systemd" boot process is in use and there is no reference left to "inittab" (e.g. if you do apropos inittab you'll probably not find anything). Instead you could do man runlevel to get similar information.

Bottom line: the /etc/inittab file is nowhere, likely because you use a newer version for Ubuntu that has a different init service, e.g. systemd.

While searching this in various forum, I got to know that /etc/inittab is not anymore used in debian distro.

It uses /etc/init/rc-sysinit.conf