flow-mask: 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/bin/flow-mask - from package: flow-tools

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

On Debian possible default full paths are:

/usr/bin/flow-mask - from package: flow-tools

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

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