flow-expire: 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-expire - 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-expire - from package: flow-tools

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

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