markdown-calibre: 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/markdown-calibre - from package: calibre

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

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