Why pssh command is not working?

Regarding the why, see /usr/share/doc/pssh/README.Debian.

To avoid any conflicts with the putty package, all of the programs have been
renamed.

parallel-ssh is pssh
parallel-scp is pscp
parallel-rsync is prsync
parallel-nuke is pnuke
parallel-slurp is pslurp

About that symlink, a slightly cleaner solution is creating /usr/local/bin/pssh symlink and let it point to /usr/bin/parallel-ssh. With the exception of /usr/local/ it is generally best to leave the content of /usr/ to the package manager. By default /usr/local/bin is too part of the PATH environment.

Try with parallel-ssh it has been renamed.

Quote from package description:

 The package contains:
 .
  - Parallel ssh (parallel-ssh, upstream calls it pssh), executes commands on
    multiple hosts in parallel
  - Parallel scp (parallel-scp, upstream calls it pscp), copies files to
    multiple remote hosts in parallel
  - Parallel rsync (parallel-rsync, upstream calls it prsync), efficiently
    copies files to multiple hosts in parallel
  - Parallel nuke (parallel-nuke, upstream calls it pnuke), kills processes on
    multiple remote hosts in parallel
  - Parallel slurp (parallel-slurp, upstream calls it pslurp), copies files
    from multiple remote hosts to a central host in parallel
 .
 These tools are good for controlling large collections of nodes, where faster
 alternatives such as gexec and pcp are not available.