itop: command not found

Have You tried accessing the binary with the full path?

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 Debian possible default full paths are:

/usr/bin/itop - from package: itop

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