calibre-customize: 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-customize - from package: calibre

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

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