calibre: command not found

Have You tried accessing the binary with the full path?

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/calibre - from package: calibre

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