clusterssh: 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 Debian possible default full paths are:

/usr/bin/clusterssh - from package: clusterssh

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

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