i.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 CentOS possible default full paths are:

/usr/lib64/grass-6.4.4/bin/i.cluster - from package: grass

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

On Debian possible default full paths are:

/usr/lib/grass64/bin/i.cluster - from package: grass-core

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

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