iftop: 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/iftop - from package: iftop

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

On Debian possible default full paths are:

/usr/sbin/iftop - from package: iftop

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

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