ntop: 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/ntop - from package: ntop

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

On Debian possible default full paths are:

/usr/sbin/ntop - from package: ntop
/usr/lib/debug/usr/sbin/ntop - from package: ntop-dbg

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

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