why-dp: 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-dp - from package: why

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

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