postbooks: 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/postbooks - from package: postbooks

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

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