v.net.flow: 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/v.net.flow - 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.net.flow - from package: grass-core

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

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