when: 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/when - from package: when

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