nxssh: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/opt/opennx/bin/nxssh - from package: opennx

You may install the required package with command yum install {package}

Error in other languages:
nxssh: bevel nie gevind nie
nxssh: comandă negăsită
nxssh: comando não encontrado
nxssh: commande introuvable
nxssh: command not found
nxssh: käsku ei ole
nxssh: không tìm thấy lệnh
nxssh: komanda nerasta
nxssh: Kommando nicht gefunden.
nxssh: kommandot finns inte
nxssh: komut yok
nxssh: nie znaleziono polecenia
nxssh: níor aimsíodh an t-ordú
nxssh: no se encontró la orden
nxssh: no s'ha trobat l'ordre
nxssh: opdracht niet gevonden
nxssh: parancs nem található
nxssh: perintah tidak ditemukan
nxssh: príkaz nenájdený
nxssh: příkaz nenalezen
nxssh: команда не найдена
nxssh: командата не е открита
nxssh: 命令找不到
nxssh: コマンドが見つかりません