userdel: 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/userdel - from package: shadow-utils

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

On Debian possible default full paths are:

/usr/sbin/userdel - from package: passwd

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

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