rssh: 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:

/usr/bin/rssh - from package: rssh

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

On Debian possible default full paths are:

/usr/bin/rssh - from package: rssh

You may install the required package with command apt-get install {package}

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