dvddirdel: 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 CentOS possible default full paths are:

/usr/bin/dvddirdel - from package: dvdauthor

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/dvddirdel - from package: dvdauthor

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

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