whatnow: 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/mu-mh/whatnow - from package: mailutils-mh
/usr/bin/mh/whatnow - from package: nmh

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

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