why: 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 Debian possible default full paths are:

/usr/bin/why - from package: why

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

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