iptraf-ng: 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/sbin/iptraf-ng - from package: iptraf-ng

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

On Debian possible default full paths are:

/usr/sbin/iptraf-ng - from package: iptraf-ng

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

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