calibre-server: 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/calibre-server - from package: calibre

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

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