v.clean: command not found

Have You tried accessing the binary with the full path?

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/v.clean - 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/v.clean - from package: grass-core

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