pick: 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/pick - from package: mailutils-mh
/usr/bin/mh/pick - from package: nmh

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

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