iotop: 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/iotop - from package: iotop

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

On Debian possible default full paths are:

/usr/sbin/iotop - from package: iotop

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

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