cluster: 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/cluster - from package: graphviz

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

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