readdvd: 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/readdvd - from package: qpxtool

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

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